What is the reason people can remove you as friends on facebook?

[Mastercream616]

People remove people for different reasons.

I removed a whole lot of people I went to high school because I really don't care to talk to them anymore. Honestly, I don't really like them and I'm not interested in what they're doing.

Also, I've added some people for an application I play. Some people might delete others if they delete that app. Or just want to cut down on the number of friends they have 'cause their list is getting too big.

I don't think there's a way to keep friends forever unless you're good friends in real life - and even then, it depends on them keeping their account - which is another reason why your friend may no longer be that.