How many of us have held on to friendships that have in actuality run their course? I'm sure we all have them. Just as we all (hopefully anyway) have those friendships that can stand the test of time - no matter how little or much interaction there might be. You can pick up the phone and start up right where you left off.
I think you have to make a concerted effort in order to keep those relationships thriving. Case in point - we moved from ND and when we moved back over 6 years later, some relationships just picked right up again. Because there was email, myspace, facebook, actual travel to visit each other, etc. There were weddings and babies - times to rejoice and those friends were there. And now we're planning a vacation back out to Colorado to keep the relationships we built out there thriving as well.
I think it's too easy for us to get caught up in our own lives and our own activities to actually work on relationships. And to me, that's just sad. Too many times you hear about friendships ending, but there's no real reason why, other than time and maybe distance. New friends come into the picture, that you might see more often because of school activities or work, etc and it is just more convenient to call them up instead of your old friends.
Now to question the other side of the situation - how long do you keep fighting for a friendship without getting anything in return?