I've stuck to the same best friend for 9 years, and I stick close to only a select few to prevent usage of myself by others. I've had many people trick me into thinking they're my friend and not just a user of me, and as nice a person I am (I hope I am), I know when to draw the line and cut people out of my life. It's a tough thing to do but it haas to be done to prevent pain.
(I had recent experience with almost losing my other best friend so it's fresh in my memory, not fun stuff)