If you obsess about something, you constantly think about it, like kids who obsess about their grades so much, they get upset before every test.
Obsess comes from the Latin word obsessus, which means to besiege, the way obsessive thoughts can besiege, or attack, your mind. You can obsess about something, like the tiny spot on your shoes that no one can even see, or be obsessed, or haunted or preoccupied by something. For example, the memory of a big, delicious dinner could obsess a hungry man.
