Why do you unfriend on facebook?

I don't often unfriend people on facebook, but when I do, it's usually because that person is constantly posting political commentary on race / religion. I don't regularly "purge" my facebook friends because I don't really want to spend the time going through my list and deciding who should be on my friends list. The thought of doing this just seems like something a teenage girl would do just to slight people they don't like. But I'm starting to think that many people do a purge of their friends list. Every so often, I'll notice that a former facebook friend has unfriended me. It usually comes as a shock because I'm not friends with people that I've never known or been friendly with. I also don't post a lot so it is odd that someone would go out of their way to unfriend me. Most recently, I noticed that a former colleague who I was pretty good friends with had unfriended me. It sounds beta I know, but it kinda hurts. I haven't spoken to this person in a couple years, so I don't understand what would prompt the unfriending, unless this person does some sort of purge of friends that haven't been communicated with in a while.

Why do you unfriend people?