"Happiness is when what you think, what you say and what you do are in harmony" -Gandhi
Yoga practice includes both physical and mental practice. One of the first goals of yoga is mental - to still the mind. A still mind is steady and calm, it brings inner peace and joy.
One of my favorite quotes from one of India's classic texts, The Bhagavad Gita sums up the power of yoga; Arjna asked, "What is the behavior of those who live in such yogic consciousness?" Krishna said, "Their minds are steady and calm. They are free from fear and anger, unaffected by good and evil. They deploy their senses only when needed as a tortoise does its limbs. Yogis transcend the life of the senses, for they know a higher taste."