Want to make your asana session more enjoyable and more beneficial? Then relax as you practice. Whether you do yoga for exercise, to relieve stress, or both, you'll get more out of it if you relax. Don't stress about how stiff you are; don't strain to get into a pose. Just do your best and enjoy it.
If you're always thinking about where you "should" be, you'll miss what's going on in your body at the moment. But that awareness is the key to practicing asanas correctly. So instead of worrying about how inflexible you are, just notice your tight spots. Watch your breath; notice how it affects your body, and let it help you open and release into the pose. Observe your mind—keep it focused on the pose and the breath. Don't let it bully you into pushing too far. It's best to hold a pose just this side of your limit. As you breathe and relax into the pose, you'll be able to hold it longer and gradually extend your boundaries.
You can even relax in strengthening poses. While contracting certain muscles, relax those that don't need to work. Don't hold unnecessary tension in your face or jaw, in your neck or shoulders. This tension blocks the flow of energy in the body that yoga is designed to enhance. And it's that free-flowing energy that makes you feel good. So relax into yoga and yoga will help you relax.