Why do I do this?

Well, one thing you should rule out is depression. But keep in mind, in the Northern Hemisphere (if you're in the Southern, my apologies for the assumption), it's drab and icky and so you might be experiencing Seasonal Affective Disorder. It's a very real syndrome. Fortunately, the treatment is light. I have a small full-spectrum light that I bought on Amazon that can plug into a USB port, and I can honestly tell you that it helps.

Is this something new or is your personality one of being the introvert. There is nothing wrong with NOT being the life of the party.

If this is something new and you are feeling more and more anxious about being around people, please seek help. It may be an anxiety disorder cropping up.

May I ask how old are you?

This answers some of the questions http://able2know.org/topic/205907-1#post-5224725