If you ask me, it's relationships. It doesn't matter how great your job is, how much money you have or how good you look, relationships play the biggest part in our pursuit of happiness.
In 2002 Ed Diener and Martin Seligman conducted a Positive Psychology study on students. They discovered that the happiest individuals where those reporting "strong ties to friends and family and commitment to spending time with them" ("The New Science of Happiness," Claudia Wallis, Time Magazine, Jan. 09, 2005).
From a very young age, romantic movies and novels teach us we need to find a romantic partner to feel whole. Once we have found someone, we put the burden of our happiness on our lover. This can create a codependent relationship where the other wants to be needed or the other person feels suppressed and leaves.
The most intimate and important relationship we have, is the one we have with ourselves. Yoga creates space for self love and self acceptance and teaches us that everything we need, lies in ourselves. Because we understand that we don't need the other, we are able to create a healthy relationship.
Yoga also teaches us to detach the ego from the self, which enables us to respond instead of react.
You understand that when the other is angry and screaming at you, he is actually hurt and crying out for attention. Sometimes doing or saying nothing and just listening to the other, is the best response. As long as we our attached to our ego, we wil want to defend ourselves, making the situation and the quarrel worse.
A third way, yoga improves our romantic relationship is by teaching us to live in the present moment. We learn to let go of past hurts and we take the pressure off for future plans because we know everything will happen in the right time. We learn to listen to our needs and wants and to fulfill them for ourselves, rather than expecting too much of the other.
If you have a lot of unresolved issues, I recommend this book that helps you resolve them with interpersonal pshychology and will better your relationship with yourself and with others.