... obsessing over a person?
I know there's general obsessing, infatuation, having a crush etc but I read on here once a word which described a sort of condition or personality trait where someone jumps from one very very intense obsession to another. I looked it up as it sounded like a friend's behaviour and it fitted perfectly. But I can't remember the name of it!
Can the MN collective help? I think it began with an F or an L (but might be wrong)