Just like working out, you and only you can push yourself and see results.

This isn't always true though. Many people who get really serious about working out have partners or small groups that work out together and push each other to go farther.

I think it really just depends on your temperament. Many people are like you and prefer to write or work out completely alone, but other people like to write or work through problems or work out in groups.

I think that, in a very general sense, writers are probably more anti-social than many other kinds of people, but humans are a social animal as a species, so I really think it just depends.

But some of my favorite works ever were literally written by pairs of people working together, like the Expanse books. I honestly think that group writing will get more popular in the future because of instant communication.

Again, to each their own though.