Everyone tends to leave or betray. Some may not even feel it when friendship fades away but it does... slowly. Friendship is just temporary.
I haven't seen a case when someone hasn't been betrayed or abandoned by an old(dear)friend.
Take for example your friends that you had in school or kindergarten.
Are they here with you? Most move on. How can you explain this? May it be because as we grow, our preference of friendship grows with us? Does everyone get tired from a permanent friendship?
How many friends have you had in kindergarten, high school, college, work(These places are the ones people usually make a lot of friends at.)... etc.?