Hi Sophia, I notice you going back and forth between two entirely different perspectives here: what you experience for yourself, and what it might "mean". I have a feeling that the former will prove a lot more useful to you in the long run.

I offer (only half kiddingly) the following hypothesis: it's partly a symptom of worrying that it might be a symptom of something.

Keeping to yourself could turn out to be bad for you, or good for you, or most likely a combination of both, depending on what it's about for you. The best way to find out is to keep looking -- and to talk about it.

I don't think it's going to make a whole lot of difference why "people" (whoever they may be) self-isolate. You seem to be asking here why you do, and I think you'll eventually find your answers by looking at what it's about for you -- as you've already started doing:

That might be true, but the only way you could know that would be by getting those very people to let you know how they actually experience you. You might, on the other hand, find that it's mostly you who doesn't want to be around them -- or both.

"Not worthy" sounds to me like depression talking. If you were to notice, more neutrally, that you sometimes didn't feel able to hold up your end of the friendship, that might be a time to get away and do something else for a while -- and/or to seek out friendships that suited you better.
Exactly. Could the "low self esteem," if any, be partly a product of your thinking you shouldn't be alone even when you prefer to be?

The people in cliques have their defense mechanisms and you have yours. You're the only one who can say how your particular defense mechanisms are working for you and whether you're ready to try something different. Just keep looking -- and posting.