Why is making friends difficult?

I'm just asking, if you have had difficulty making friends, what has been difficult about it? What has made it hard for friends to stick?

I don't have that many friends, but I do have a few good ones. I met most of them through one particular person. If I didn't know him, I don't know that I'd be friends with all the people I met through him. So I kinda feel like I got lucky. I think I'm most held back by social anxiety. But I definitely rely on other people for social situations. I never initiate anything.