lol, that's literally how I got my first friend (kindergarten). It might be a false memory, but I remember it being that simple. "do you want to be my friend?" "sure".
I also clearly remember a girl asking me the same question in class, but I was so shy I didn't even know what to say, so I never answered. Just pretended I didn't hear her ask.
I know this is going to sound really lame, but I remember reading "How to win friends and influence people" maybe when I first went to college, and found some of it very helpful. I now know most of the stuff that was new to me is actually common sense to most people. I think the advice about being "interested" in whatever the other person is saying / doing is really on point. I still do it sometimes when I talk to someone and I start asking questions, then basing my next questions on which questions they were more enthusiastic about answering, etc.
The only problem I have with this approach is that it only makes a relationship more of what it already is. So, if I use it with a coworker, we become better coworkers. If I use it with a FB friend, we can chat more, because it's still a conscious thing.
I think my experience with close friendships is similar to what
was saying. I just felt naturally more comfortable around the person. I wanted to spend more time. I wanted to listen to what they had to say about everything, and when I talked about myself I never felt guilty about it because I wasn't "really" talking about myself. It was always something they enjoyed listening to. No one was putting effort in anything (when it comes to spending time together anyway).