I Don't Get It

I never understood those people who have a revolving door of friends. You know the type who you become "best friends" with. To only six months down the line have your calls and e-mails ignored, never returned without an explanation? I don't get it. Sure people grow tired of each other. But friends aren't the latest clothing accessory. You don't throw them out just because the season has changed and the fashion is different. Growing apart i understand, but serial friendships i don't.