If I'm working on a song, I always want to do iit by myself. With help, but I'm not at the point where I"ll do a song with someoone yet. Why?
Works of art can be very personal. In some cases, for instance, I'd personally feel a little uncomfortable doing a piece of writing with people. Maybe it's because of the sometimes-personal nature of creative work?
Finding a creative partner is like finding a romantic partner only in some ways harder. Not only do you have to get along with one another, but the way you think has to both play off one another and gel in order to have a successful team. I can think of only two people I would choose to work with off the top of my head, and that's after many years.
I know what you mean. If I'm working on a piece that's meant to involve more than one person, I choose my collaborator(s) very deliberately. You can't make any kind of art with just anyone. You need someone who's going to be honest with you as well as encourage you and spur you on. You need someone with whom you can work well. They should also really be contributing, not just doing little things worth mentioning in the liner notes.