Psychology has a lot to say about the experience of outgrowing someone and perhaps losing a close friend. It's an emotionally complex realization but you cannot possibly keep every friend you've ever had. When we see someone every day or regularly for a period of time, our neurons become wired around their presence. So when those relationships begin to fizzle, our brain cannot respond as it has previously. This causes that sense of confusion and that sense of hurt. We may find that outgrowing a friendship involves a similar five-stage process to grief. You have to go through hard things to enter into a new chapter to see the other side. But sometimes that's just the way that life goes.