The passing of friendships needn't be mourned -- such events are all part of the cycle of social life. As a Navy brat, I moved around a lot as a kid, every few years leaving my school and friendships in one town and socially starting from scratch in another. From this I developed (as a sort of protective mechanism) the ability to part with ease -- I don't feel the need to collect and hang on to people like magazines to be piled in the garage.
I'd rather spend time lavishing care and attention upon those cherished friends whose value to me continues to grow with each passing year. Not only is my life thus enriched, but I am also sparing my exes the discomfort of having to exchange fake pleasantries and force smiles, leaving everyone more time to seek out and nurture the next great friendship.