"I always used to be serious about the yoga activities I do,"
When you do Yoga, try to invite imagination with arts. It will not only make the postures come alive but force your mind to enjoy the activity. Being with a group is truly a fun and when it is about Yoga group, you have way lot of things to try. My yoga group is a real fun, we try hell lot of poses. We laugh, we go out, we celebrate and what not.
So, workout is never a serious activity. If you will take it seriously, it will become compulsion on you so try not to be formal with it. As and when you will start enjoying, it will become your part automatically.
The amazing fact about yoga is that it essentially addresses all of these modes of expression; from partner postures, to chant and humming, to knowledge about angles as they relate to the body and our bodies as they relate to the natural world.