I have had sooo many people that I thought were friends just not put in their 50% of the relationship. A friendship, whether it be just a friendship or an actual relationship, should never be one sided.
I just recently went through a situation where I realized I was the only reaching out to someone who had once been a great friend. The decision was extremely difficult, but I decided to cut my ties with that person. I figured if they realized they still wanted to be friends with me, they would call and put in more of an effort. And guess what...I haven't heard form that person so I'm pretty sure I made the right decision by cutting ties.
That's not the first time that's happened to me and I'm sure it won't be the last. I do know that surrounding myself with people who do call me and invite me to do things has shown me a whole new side of friendship. Before I had entered college, I was always the one who planned things or called people. Now I still call people, but others take part to plan things too!
I find my biggest problem is that I grow up and people who I was once very compatible with just don't. A lot of them decided not to go away to college and are stuck at home doing the same things they've been doing for the past 20 years. There's not a whole lot of opportunity to grow up in that sort of a situation! Therefore, I meet new people on my level and I drift away from them. Its hard, losing friends...but I've come to accept it as a natural way of life.