This is a tough one for me too. It depends on where you live and your culture which does affect friendships. The Western cultures favor individualism more, so are individualists before they are friends with some exceptions.

Life often pulls people apart. A lot of my friends have families, and I'm still single. They have their own pastimes and responsibilities, and are often more comfortable spending time with other families.

People can move to different places, and even though there are often promises to keep in touch involved, most of them are not kept. Generally, long-distance relationships of any sort don't last very long.

In addition to this, sometimes people change internally and they have a need for new friends. I know this happened to me. I felt bad about not hanging out with my old friends, but doing so reminded me of the old me that was changing into something new.

It is best to learn to make new friends by placing yourself in situations where you can meet new people. This way you'll feel more confident about moving on in case your current friend pulls away for whatever reason.

Otherwise, try to shift your focus to a hobby or another activity without pushing these thoughts away. Just don't focus on them. If you push them away, they'll keep coming back stronger and stronger. If you don't, and simply go on with your day without focusing, they will eventually weaken.