Over time, friends may leave your arena for all kinds of reasons. Once way back, I left a friend's arena. I didn't even think too much about it--but in hindsight, my leaving was probably interpreted as cold and uncaring. I was single focused at the moment and my friend didn't fit into that single-minded direction.
Friends may leave your arena because of illness, distance, new interests, new relationships, goals, or even something you may have said or done--something you may not even be aware of.
I've read a lot about this since I'm a sensitive person who likes to stay close to my friends. I'm also an honest and direct person so if something is bothering me, I'll use let people know. I realize that not everyone is like that, and many are not as forthcoming as I am. There's good reasons to be forthcoming, and there are not so good reasons for that. As with all things in life, a balance is usually best.
As I look ahead, I'll continue to foster good relationships with family and friends to the best of my ability. I'll also challenge myself to be a bit less sensitive and bit more resilient as develop, change, start, and sadly, sometimes end. Onward.