They just do, trust me I am one.
It's a big question to ask and there's not really a definite answer...
If you're cast out by society it could be for a bunch of reasons, socially unacceptable behavior, being a deviant in some way, though deviation from the norm isn't always bad people don't always like it.
You could have beliefs that people don't agree with or have some problems interacting with people, a lot of things could cause you to be cast out of society...
You could also have trouble with interaction and so avoid it, becoming an outcast by your own actions.
A lot of people find themselves incapable of "fitting in" during different stages of their life and as a result feel like outcasts, if a person has really bad trouble fitting themselves in to a society for a long time the may choose to avoid being part of it, alternatively they may become social pariahs for the same reason.
To give a rough answer, extreme deviation from social norms can cause people to "cast you out" as it where...
The whole thing is far more complicated than all of this though and it's important to recognize that there's a huge amount to this...
Nobodyinparticular posted links to ostracism materials, worth reading up on if you really want to get in to it a bit more.
Every case is unique though, so generalizing is a complicated issue.
That is a very complicated question that would take books to answer completely. You can start by reading the Wikipedia articles associated with social ostracism. You may also find helpful this sociological paper, Ostracism in Small Groups: A Theory of Hostile Ties (scroll right to see it).
Because they are different, and the majority of people are afraid of different people?
But I have no friends...oh
they have no friends