Friend or doormat? Where's the line?

As an only child, I treasured my friends from an early age, and over the years my closest friends have truly become like family. Needless to say, I consider my friends incredibly important in my life, and I believe in loving them unconditionally. In the last year or so, though, a couple of incidents have forced me to think long and hard about what friendship is--and isn't--and when it's time to end one.

Case #1: A friend from many years ago has been in and out of my life for the last two decades. She was never one of my closest friends, and I often felt like her idea of a friendship was having someone to listen to her problems rather than having a relationship based on mutual trust and support. As much as I value my friends, I began to feel like I wasn't valued or respected, so I pulled back from this relationship somewhat suddenly.

Case #2: Another friend has needed lots of help--financial and otherwise--a few times. But again, it started to feel like she only contacted me when she needed me for something, not when she just wanted to hang out. I let it go, figuring it was just because her life was in crisis. Finally, I needed a favor--not one that required a major sacrifice on her part but which was nevertheless really important to me--which she was in a position to grant. I asked, and she agreed. At the last minute, she bailed with a lame excuse. Time to end another friendship?

On the one hand, I don't believe the value of a friendship should be based on what you get out of it. That's too much like a business transaction. I also don't believe in ending friendships casually over some small slight. Instead, I believe that friends should be there for each other in bad times as well as good, listening to each other and supporting each other. Friends should forgive each other for small slights (and for big ones too). But when does "being supportive" become "being used?" It seems to me that true friends are there for each other, not because they feel obligated but because they want to help each other. They want to spend time together because it's fun, not because they need someone to dump on or someone to help them out of trouble. I contrast the two examples above with my two closest friends. Both of them have needed my help at one time or another, but both of them seek me out whether or not they need something. And they've both been there for me more times than I can count.

So am I kicking people out of my life because I don't get enough benefit from having them in my life? After much reflection, I don't think so. Instead, I think I'm learning to make better use of my limited emotional energy by not wasting it on people who care only about how they can benefit from having me in their lives. Yet sometimes I still wonder if I'm just being selfish.