I've felt that way once. It wasn't exactly love, but the moment I saw him, it was like all the air in the room disappeared. Sort of like in Twilight when Bella walks in the room for the first time and Edward suddenly looks like he's straining to take a huge poop. Only hopefully I didn't look like that
We briefly dated, I was crazy about him, but he wasn't that into me. I fantasized for a loooooong time about what our life would have been like together. Luckily I've moved on and am very happy, not sure what he's up to.