I've experienced it once. The only thing that stopped it was getting together with my dp, with whom I have an extremely balanced, calm and delightfully non-obsessive relationship 
It went on for years, however. It was absolutely awful. I'd been with him for three years, first love. I actually ended it, but became completely obsessed with getting back together, for years and years afterwards. He was actually a very nice guy, not at all an evil git, but I'd hurt him and he didn't want anything to do with me (understandably, as I'd behaved v badly at the end). And that just fuelled it until I was in this state of madness.
I spent hours and hours internet stalking him. Every time I got on a tube I'd check up and down the carriage wondering if he was on it! I'd plan trips to places I thought he might be (thankfully I never actually went on them). I used to hang around our old flat, just walking around and around, thinking about him. I'd cry and cry and cry, and obsess, and go to bed every night and lie there conjuring up scenarios where we'd meet. And I'd dream about him a LOT, which was horrible. Just on my mind, all the time. Literally all the time.
It did fade a little, but it didn't really go away until I met dp. Even now, I do think about him quite a lot but it's just 'oh, yeah, him' rather than the awful obsessive weirdness of before.
I do think it's a genuine mental health issue. I did feel as if I had lost my mind - I knew I was behaving oddly but I just couldn't stop it. I hated being that way and am so pleased that I feel 'normal' again. It did all escalate at a time where I had lots of other crappy stuff going on and I have often thought that these obsessive thoughts and fantasies were a bit of an 'escape', almost a safe place I could go to, even if it made me feel bad in the long run. Like an addiction.