It takes a lot of energy and will to let go of relationships.
Whether this be friendship, family or that "significant other".
It gets harder and more dreary as you get older where you have to let go of multiple relations due to responsibilities and a lack of compromise and commitment.
In elementary school when my "best friend" (after knowing her for only a year) had to move away to Catholic School, the situation seemed simple. I did not think about the distance and minimal contact we would have that would effect our friendship. To be naive of the entire situation and hold on to that make believe that we were "best friends" eliminates all expectations. We went with our daily lives without seeing or talking to each other days at a time. Sure its not about the quantity but the quality, though there's a point where you are not as significant to them as they are to you. Now, when you reach this point where you realize you care more then the normal individual where you become morally conflicted whether you should continue to care. Is there any wonder why its inherit that we are selfish?

"I would die for you. But I wont live for you" - Reference to The Fountainhead