There are no set rules per se. You can follow anyone that you like for whatever reason: they're funny, they're helpful, and such. People will follow you for the same reason. While it may be nice to follow someone that follows you, you do not have to. I try to, but, based on some users' posts, I don't always follow back if the user seems to only talk about sexual topics for example.
Depends on your reasoning why you'd follow someone... people on here are different so you can expect a mixed opinion of which is a valid answer, but in my opinion I think people largely follow those that peak their interest on this site.
Following a member simply allows you to be more "in touch" with that person. If you choose to, you can filter the content in your Live Feed by users you follow so you will only see their activity. If you find a particular user helpful or simply like their posts, you can follow them and this will help you see their posts on the site.
In some cases, following can allow you to private message other members too if they have their settings set to only allow messages from users they follow. This information is available in our FAQ as well. :) http://www.girlsaskguys.com/faq#75