Hi AIM,
When I go to a thread and read it and don't reply it's usually because I don't have time to reply, or the story is not something I can really relate to so I figure it's best that other people reply, or because something on it is triggering me so I avoid it. It never has anything to do with the other person and what they say, since I do sometimes reply to threads that are difficult to read. I'm replying to yours because you're my friend.
I have all the things you listed (I'm popular, have lots of loving friends and family, very successful at my job) but I still get very depressed. There's no formula. But longing for something else, to be out of our body, to be anywhere-but-here is a symptom for me that comes and goes.
Take good care
RJ