It's not like I "break" friendships in the strict sense of the word. It just seems like every year, I mature a lot and, therefore, grow apart from old friends and get new ones. My maturity level has changed drastically since my Middle school and Freshman days (more drastic than the average person, I would imagine), and so have my interactions and ideas of social acceptance. Kind of a "it's not you, it's me" sort of deal.
Not like I've ever had any particularly strong friendships in school; they were all just interactions at school exclusively. Outside of school, I'm pretty much alone.
Online is the same kind of deal: different friends every year, exclusive to the public place of meeting (forum threads in this case). Although, recently, I've been able to establish firm, one-on-one friendships with others that I haven't broken yet.