In my opinion the difference between desire and obsession can be found in self love.. if you love yourself you are happy in your present state of being. In this state your are able to feel desire for someone: you meet someone and you feel happy to be with them. But it's not making you crazy when they don't text you back for an hour and you can continue your life looking forward to see them again. You also feel that they return your feelings and you have trust in this relationship. If you think of yourself as 'not enough' (to be loved e.g.) you become easily obsessed with another person and you tend to put them on a pedestal, you only think about them 24/7 and you can't continue living a healthy life. The fact that you put someone on a pedestal shows that you belittle yourself. When you realize obsession you should work on yourself, shift your focus and see your own self worth.

I am german sry if there are any mistakes;)