Mainly because when I googled following relationships in rails, Michael Hartl's tutorial said so :-) -- it's googling so I count it as my own work!

When a user follows or unfollows another user you are basically preforming queries and adding rows to tables -- objects talking and doing stuff to other objects. When you create the relationship in the controller, you are essentially telling a user object to do something with an association to another object. The only thing the controller is suppose to do is coordinate the event, the timing of when a user creates or deletes a relationship.