I used to be too serious. For me at least, it was a self esteem thing. I had very low self esteem, so I was always anxious and tense in social situations. This made me act extra serious, afraid to smile or laugh or flirt.
This may or may not be your problem. You have to ask yourself why you have this problem. Do you take things too personally? Are you afraid to smile? Are you easily offended by "offensive" material? Are you having fun with life, or are you too focused and letting the little joys pass you by? Whatever it is, find a way to deal with it. Let go, relax, and enjoy the ride.