My mates at uni are as crazy as me, so I'm not gonna lose them if possible, but I wouldn't say they're friends for life. 2 of my best friends I've known for 90% of my life and 1 I'm just extremely close too. My normal mates are either friends of one of my best friends, friends from secondary school, or snowboarding buddies.
My uni mates are just crazy fools who drink too much and don't think enough. Pretty much the same as me half the time, which is why it's great, but it's not something to last a life time (though it may kill me!!).
It all depends on the people, but the general thing is that when you live with people for 2 or 3 years in a house, you become really good friends with them, and that friendship won't just disappear.