The right fit

I used to think I'm quite flexible and not-so-choosy when it came to making friends with people. Here, however, I've come to realise that it's quite the contrary. You just can't stand some people, while with some the closeness is more of a sine wave. Even the select group of people, the so-called close friends drift apart at the minutest of causes (or even without them), their void getting filled (though slowly) by others.
Why is it that school/college friends stay dearer than any latter ones?
Is it the idiosyncrasies attached with growing up and becoming more opinionated? Or is it the constant urge of people to mold others in their 'model' casts - something at which they continually fail? Hard to say...
An operations guru may say that the most optimum way to deal with these is to Stop Caring. Sadly, ops ain't my cuppa tea...