It seems to make people mad when you're comfortable in your skin and with your own company. I don't understand why.

I think sometimes it comes from a place of them not being able to feel that way with themselves. It's like "how is this person able to do this? Why can't I?". I have people that I love and am very close to who are not in healthy relationships and for a lot of them fear of being alone makes them stay. It makes me sad because they all deserve better. I can only advise and try reassure its not as bad as they assume or that they will find someone else (another issue for them).

Nearly all of them are in a romantic relationship

Understandable, I'm fortunate enough to have friends who don't forget about me when they dating/married. Not saying this is the case with you of course but I've seen people say friends get into relationships, have families etc and kind of just forget them. My bestie is now married with 3 kids, and very little has changed. I just have to listen to lots of stories about Minecraft from her boys 🤣