Focus is paying attention to something and making that your prime thought. It can be seen as a lasso. When you want something, you can throw your lasso around it and pull it towards you. Focus does that. It is a tool that brings you closer to something.
Obsession is an extreme energy that occurs when someone cannot get rid of their focus. The energy that they have put into their focus is overly strong, making them infatuated. Using the lasso analogy above, with obsession, the thing that someone is trying to rope gets repelled by the extreme energy trying to grab it, so it runs. A person in this situation ends up being dragged by the very thing that they are trying to pull towards them.
There is a fine line between focus and obsession. People who are obsessed have a hard time engaging in conversations because their mind is unable to concentrate on anything else. All they have to do is let go of the lasso, and completely drop their focus on something they are obsessed with, in order to return to a balanced neutral point.