I have always queried this question many times in my head, and so I thought if I wrote it all down I may actually have an answer to the question.
1) He has no conprehension about relationships and the reason for them. Now I sometimes am the same when it comes to these and I wondered myself if I am an alien but have proof that I am not because my Mum told me that she had to have a drug to ease the pain as she gave birth to me..... so that answered my question.
2) Sheldon doesn't know how to utilise the basic human functions in social situations like a greeting for a civil conversation without making the other person feel like they want to kill him; I'm not very good in social enviroments but that's because I'm an introvert and become very drained when I'm around people.
Also, I don't enjoy parties because I stutter and literally have no idea what to say to people who I don't know.
3) He is obsessively enthusiastic about the fantasy and sci fi world so I'm thinking that he is subconsciously trying to connect or familairise himself with fellow aliens and their life stories. But then, I love lots of geeky things and I know a lot of other people do too and we can't all be aliens as I believe if we were then we wouldn't look like human beings which is another question I pose; why does the Doctor look human?
This may be a pointless post and a complete waste of my time but I wanted to share a little piece of my mind and what I think about when I'm alone.