Why do people on facebook hide their friends list?

[Smartass Italian]

Just curious, I see this a bit here and there, like when I friend someone from school or such and try to see if they have other friends from school I can add and all I can see is mutual friends. My girlfriend also has her friend list hidden. Are people trying to hide something or someone? Why hide who you are friends with?
I am seeing some decent answers so far and this doesn't look good for the girlfriend. It seems as if she wants to hide ex boyfriends from current boyfriends (me) and keep them around so she can know what they are up to and keep tabs on them. And also she can like, comment and chat with them and I have no idea who they are and what she is liking, commenting or saying to them. Pretty sneaky tactic and causes a reason not to trust them.

[Cone]

My cousin used to hide her friends too..
Its to prevent people from scaring your other friends away if you get into a fight or something, and to prevent people talking crap... like o wtf, ur friends with that idiot.. ect

[Thushyantha]

when someone hiding friends list from their Facebook profile means 100% they don't like to share anything with anyone and hiding many things.. not only in Facebook real life also they will be like that.

[cherrystarburst]

just for privacy reasons.

[Mike Toddz]

There may be several reasons to hide friend list for example I'm hiding my X girl friend from new one. I'm hiding my old friend because my close friend doesn't like him etc. So stop thinking and hide your friend list too.

[R]

They hide them becuase if you are a friend with someone and you really don't like them but want to keep them on your facebook they wont see anything at all you post/ comment and like