I know what you are talking about, I think. Why do you consider people "friends" even though you only see them once every couple months, or maybe even a couple times a year. You don't share your daily life with them, but you see them whenever you go home for the summer, or winter break.
The conclusion I have come to is that its the people you see every month or two that are the lifelong friends - they're the ones you share life events with. I haven't seen two of my closest friends in over three weeks; not because I don't like spending time with them, not because there's any lack of desire to spend time together, but because we're just busy.
I have a job that's taken away my evenings for the last two weeks; that's the way it goes. When this coming year hits, I will have a schedule that holds me to being busy 8AM to 5PM three days a week, 8AM to 7:30PM the other two, and evenings one week a month. When you come home after a day like that, the last thing you want (generally) is to go out and see people. I think that friendship time gets saved for weekends and truly special occasions.
Most people, once they reach a certain age, retain most of their basic life characteristics, mannerisms, ideals - the things we find in friends. Barring any major life events, your friends aren't going to change much from one month to the next. If you base your "deep" or valued friendships off of the people you see in a business or school context, you will never find them.
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