What is the nature of friendship?

I’ll be the first to admit that I am a terrible friend (you could also insert uncle, brother and son here).  I think that it is because I am both a severe introvert and fairly lazy.  These two traits combine and negatively reinforce each other.  I’m uncomfortable reaching out to people and bad at forcing myself to make the effort... even though I know I should and I truly care them.

I’ve never been good at going out and putting myself into someone else’s life.  The times I’ve tried I’ve usually bungled it and probably just come off as to clingy or socially inept.  And I’ve always been one of those guys that isn’t invited to things... not that I’m unwelcome or actively excluded, in fact when people find me at some social gathering they are at they are always nice and seem happy to see me, its just that well... why invite him.  Of course a lot of that is because I tend to avoid social gathers and I don’t invite others into my life so of course there is going to be a lack of reciprocation.