What is the psychology behind someone who's unable to sustain relationships?

[Von Trapp]

I have never been able to sustain a single relationship in my life. Be it friends or relationships. People would come into my life and leave very often. I guess along the way I got used to it. Why? Is there something wrong with me? I had a problem expressing emotions growing up if that contributes to anything.

[Us]

Personality problems/disorders, their upbringing or social examples, etc.

[David]

Eh, people come and people go.

I grew up in military family and didn't have any friends that lasted more than a few years if I was lucky.

I kind of grew up normal.

[Captain S]

Many causes will produce the same outcome, so it is impossible to identify exactly why this is with you without further information/insight.

[Dick]

Not even with twitter?