And one completely (but the most important) unscientific reason. Get to that in a second.
Put First things First
When you start Ashtanga Yoga one of the first things that freaks people out is the start time. I personally start at 5:00 am but most of our students at Ashtanga Yoga School Jasper roll in just after 6 AM. Often times leaving a led class on a Friday at 7:30 before the world has even had a cup of coffee I ask myself - "what the hell are you doing this for?" . The answer is because it brings me peace, health and happiness. For most people the first thing they do in the morning is work. They are putting making money first. When you commit to Ashtanga Mysore Yoga you are putting your health and peace of mind first. I always knew intuitively that after I did yoga each day I was happier and overall the quality of my life has never been better (16 years in). Then I happened to read an amazing book by Sonia Lyubomirsky called the How of Happiness. She has studied what is scientifically proven to make people happy. Out of the 12 activities that will make you happier 6 of them are cultivated when you practice Ashtanga Yoga. Here they are -
1. Increasing flow experiences
"Flow is a state of intense absorption and involvement with the present moment" - Sonia Lyubomirsky from The How of Happiness.
This is pretty obvious after your first class. Especially if you are lucky enough to get a good breath focused teacher that teaches you the traditional vinyasa count. Every movement right from the beginning of the first Surya Namaskara A (sun salutation) is linked with an inhale or exhale. So off you go through the series inhaling and exhaling watching your breath and in the flow. There's a reason marketers called it "flow yoga" when they turned yoga into a business.
2. Nurturing Social Relationships
This one might not seem so obvious at first. In fact, it can be quite the opposite. Since there is such a focus on breath and going inward, unlike western yoga classes a typical Mysore room will have very little chit-chat. Passing glances as you leave or enter the shala will be extent of the interaction with fellow students in the beginning. Just wait till the first social event though. You get a chance to talk about bedtimes, diet plans and postures till your heart's content. Kidding aside, the Ashtanga community is filled with amazing people. It's awesome meeting yogis that you have the practice in common with and having friends that are making happiness and spirituality a priority.
3. Practicing Religion and Spirituality
"Think of God. If not God, the sun. If not the sun then your parents.” Krishnamacharya set only one condition explains Miller: "That we acknowledge a power greater than ourselves.” - Yoga Journal May 2001
Yoga is a spiritual practice. From the outside it looks like some type of exercise but when you practice it you know that it is much more than meets the eye. This is the real reason why it has become so popular. Spirituality is not something you need to know or learn. It's already in you. We just need to remember it. The power of coming into your breath merging the inhalation with the exhalation allows you to access the part of the practice that is much deeper than the physical.
4. Taking Care of Your Body (Physical Activity)
It's almost impossible not to get whipped into shape with this practice. It builds strength, flexibility and stamina when you commit fully and do it 6 times a week. The commitment to a morning practice forces you to make some life decisions as well. You have to choose between staying up late eating and drinking or getting to bed and getting up to practice. At first it can be hard but you start to feel so good after the practice that you end up choosing that feeling over the activities you used to indulge in. Make no mistake, if you stick to this practice you will be stronger and healthier than ever before.
5.Taking Care of Your Body (Meditation - Eventually)
People often confuse meditation with sitting quietly. Meditation is not something you can do. It is a state that you are in once the first four limbs of Ashtanga Yoga have been cultivated. So after practicing the yamas and the niyamas (principles of right living), asana (physical postures) and pranayama (breathing exercises) then dyhana (meditation) will arise. This is not to say you won't be training your mind in these first four limbs. This is exactly what you will be doing. You are using Tristhana (breath, postures and drishti) to focus on one thing, one posture at a time.
Studies show that cultivating the ability to pay attention is the number one way you can create happiness. 
More on this in a second.
6. Committing To Your Goals
I can tell you first hand that having a goal of getting as many postures as possible doesn't work that well. The body needs time to change and adapt and you have to allow that to happen. Sharath Jois often says that the way you get the next posture is by perfecting the one you already have. What I have found to be a worthwhile goal is " just show up to practice everyday". That's all you actually have to do. Sharath says if you don't have time to do the whole practice you do 5 Surya Namaskara A, 5 Surya Namaskara B, Halasana (Shoulderstand) Sirshasana and Padmasana. It takes 15 minutes. When you commit to a goal of consistency over quantity you'll find that the ability to do the postures takes care of itself.
In Western business yoga classes the format is doing new postures with new students which by necessity results in constant talking. The result is your attention is drawn outwards and no chance to calm the mind or master the asana.
This approach just scratches the surface of what yoga can offer the student. In Ashtanga Mysore Style we repeat the same postures daily which allows you to work at your own pace, go deep into each posture by making it second nature but most of all practice in silence with a focus on your breath.
"These days It is no longer an issue whether the Chinese philosophies and the yogas are correct, it is just how do they work" - Master Herbalist Ron Teeguarden
1 Completely Unscientific Reason Ashtanga Yoga Makes You Happy
The yogis that developed these systems for health, happiness and spirituality lived in a time when the mind was much more calm. They were more in tune with the natural world and the principles that governed it. Today's world is different. We spend our time in cities, on computers, insulated from the earth and shielded from the sun.
We aren't able to hear and trust our intuition on how to live. Instead we have to look to "double blind peer reviewed scientifically proven tests" before we believe something to be true.
I discovered for myself that the real way to happiness is through yoga. Why?
The only way to happiness is to stop looking for it. - Chuang Tzu
That's why cultivating the ability to pay attention is the number 1 way to achieve happiness.
You become present.
You stop searching.
If you have read any of Eckhart Tolle you will recognize this line of thinking. He teaches how there are no problems in the present moment. Only what is.
I heard an interview one time where someone asked Pema Chodron why we can't just "be present". She said for some people that is possible but for other's we need teachers and practices to help us.
We need training.
Mind Training Vs Mind Entertaining.
The thing about training is that it can be challenging and uncomfortable. Something most of us want to avoid. Would you agree that if we are looking for a specific result then it makes sense to choose the best method to achieve that result?
The purpose of yoga is to calm the mind so you can think/hear God/Muse/Source - choose what word you want.
Somewhere along the way as yoga became popular in the West it went from mind training to mind entertaining and ego strengthening. What you see now is led classes with a teacher talking, music playing & people comparing what they can do physically.
This is not the best tool for our purpose of calming the mind & the ego to become present and hear God.
You want to know what is? I found it.
I found it by learning the method from the source of yoga in India from my teacher Sharath Jois.
I found it by having my ego crushed by losing all my postures to injury.
I found it by spending 16 years gaining day in day out "mind training".
I share the whole story in the email series below or if you live in Jasper you can contact us here
If you want to read Sonia's book you can find it on AmazonThe How of Happiness: A New Approach to Getting the Life You Want