On the tv, in books, films, magazines you will find passion expressed as love. That intensity is natural in the early stages of a relationship and dissipates as times goes on. A long term, healthy relationship will evolve into a steady, caring one.
Some people are hooked on that intensity and will seek a new relationship when that intensity dwindles as they believe they are no longer in love.
Passion can turn into obsession when the relationship becomes lop-sided...one person wants more than the other person does. Obsessive thinking can feel compulsive and can lead to relationship/sexual addiction so if you think you are obsessing seek help.
Obsession can also lead to stalking which is very unpleasant to experience for everyone concerned.