Friendships old and gone

I had a lovely brunch get together this morning with three friends that I have known since high school (just a few years). It got me thinking about friendships and why we hang on to some while others fall by the wayside.  When I think about it, almost all of the people I count as good, close friends date back to my high school and college days. I have some friends who, when through circumstance, I haven't spoken with or seen for months, sometimes years, I can still "pick up" the friendship and know that it will be as if there was no gap in time. Others, people I've worked with and felt very close to at one time or another I just seem to let go. I'm not proud of that.  I feel sometimes that I should stay in touch with everyone but, except for Christmas cards, that just doesn't happen. I don't think it means that friends I've lost are less likable, or undeserving of my friendship.  I always enjoy meeting new people but I suspect that I have a limited capacity for real friendships.  It has occurred to me that many of the people I don't stay in touch with also don't stay in touch with me. I'm guessing we all have friendship limits.