People aren't "supposed to" automatically follow back anyone who follows them. It's not what the "following thing" is supposed to be.
If someone follows me I'll usually check out who they are and what they seem to write about. If it looks like they write about stuff I'm interested I may follow them back. If they write about stuff I'm not likely to read, then I don't follow them back.
One thing can be, though, that after you've been on here for a few years you can start building up a lot of people that you're following. Because of that, these days I really have to think the person "specializes" in the kind of stuff I'm interested in reading (as opposed to when I was following fewer people, when all I'd need to see is a Hub or two that interested me). Since it gets to where you can't read everything that everyone you follow writes anyway, who you choose to follow back and who you don't can kind of become (sorry to say this) "a willy-nilly" kind of thing that's related to time and circumstances.