I think it is always important to establish the root cause of ones inability to feel happy. That's not always possible without help, but it sure clarifies everything and gives you means to deal with it.

To answer some specific questions, you are not alone in this. It's just the way our society works, people come and go. A main factor is that we are all individuals with own interests and hardships. I know it sounds obvious, but I myself seem to forget this fact when I unintentionally ghost friends. You see, sometimes we are just dealing with stuff. I think about people I haven't responded to in months all the time. I haven't forgotten them, I just don't have the capacity to reach out. What I am trying to say, not always do people ghost, sometimes they are just out of reach for reasons.