I think friendships form on HP for the same reasons they do anyplace else online or, for that matter, in person. When I like someone's writing (both the style and the substance), I tend to read more of that person's hubs. When there are a lot that interest me, I'm more likely to follow that Hubber. This is because I like the way that person thinks and writes.
Also, reading comments made by Hubbers (not only about my hubs, but about those written by others) shows me how that person reacts to topics, whether or not he or she has a sense of humor or gets easily rattled by a controversial subject. I can generally learn a lot about that person's personality and character from reading comments. It's similar to having conversations with a person who is physically sitting or standing near to you at the time. People give away a lot about themselves when they talk or write informally. (There may be many Hubbers who avoid me for that very reason!)
I don't follow a huge number of Hubbers--only those whose writing interests me and influences me to want to read more of their work. However, I don't think "following" need be limited to "a few" people....If you can (and want to) read everything written by fifty hubbers (and have time to do so), that's the right number for you to follow.....It's a personal choice, like so many others in life. If friendships form during this process, that's icing on the cake!