I have a lot of casual friends I go out with and have fun with, but they don't completely know me entirely because I perceive it unnecessary to share my entire self with everybody. They are more like in-the-moment friends. I wouldn't call them if my dog died or my car broke down. These friendships fade in and out. I wouldn't call them shallow, just connected on a different wavelength of self disclosure. I've had more intimate friendships that faded and ceased, not due to any conflict, but merely growing apart. I had a female friend from work who was in her mid thirties and getting divorced, and for a few years, we constantly talked and hung out and got crazy drunken tattoos together and stupid things like that. She got back with her husband and our communication began to fade until its been two years since we talked. It's bittersweet to think of her. I understand she is in a new phase and there isn't much room for me there--she needs to focus on her family. So while I miss her, I'm thrilled she has all she ever wanted and if we reconnect again when we are on a similar wavelength, great. If not, it was great to experience life with her for a time. Then I have rock solid friendship. Only one of my friends I consider this solid because we've been friends for over a decade, and through separations and trials, we have always come together again. She knows things about me I didn't even have to tell her--because she gets me and accepts me and visa versa. We just know. Sometimes, I'll have a thought or wonder something about her out of the blue and she will call and tell me about something going on with her life along those lines. But it's so rare for me to have a friend like that. She is the only person besides my mom I trust with everything.
The point is that real understanding and friendship is rare. Not everyone you know or meet is able to connect with you on that level--but you can still maintain some type of relationship. It takes years and years to get to the deeper more solid and intimate relationships. its all about mutual investment to get there though. so it is necessary to have a strong relationship with yourself to know when and with who to share and how. And it will maintain your contentment with life and a sense if psychological well being for those times you think you are alone or that a friend has left you. I wouldn't say my older married female friend left me-- I'd say she embarked on a new path in her life and I can cope because I know i have my own path to travel.
life is not about holding on to people, but about appreciating the time you have with them and then letting them go gracefully. We all go in the end.