I'm not an obsessive kind of person, am quite happy not in a relationship and like my own space.
I think everyone is different, some like the thrill and passion and all that, some are happy with a good partnership regardless of the sex side of things, I imagine some people just don't want to be alone - whatever the relationship or feel they can't not be in a relationship or for religious/cultural or other reasons (staying together for the kids or through serious illness etc...), though I think an obsessive kind of relationship would be hard to maintain with real life and responsibilities without something else giving somewhere or a potential big fall.
I have noticed, when seeing others be all almost fantasy world about someone at the start of a relationship that it is more likely to fall apart, often with a lot of drama.