The difference here is in how the action is perceived. In my head, someone who is "fixated" on something is very focused, but in a healthy way. Another good word here would be 'driven' or 'dedicated'; all of these words generally have a positive connotation. Obsessed individuals compulsively think about something and can't help this, so this is considered 'unhealthy' and has a negative connotation. Example: Chris is very fixated on becoming a doctor. He's wanted to be a doctor since he was a kid. Chris is obsessed with cars. He only wants to talk about them whenever we go out for food.