I covered my opinion on that here… Not saying this is what it is but its what it is to me. I think it can mean different things for different people though.
Basically Love is your natural state before being human jades you LOL
Oh my goddess, this is gonna be embarrassing sort of LOL
I knew my husband on and off all though my childhood, just from school and seeing him on the playground and stuff LOL. We were not close or in the same circle of friends till high school though. I had no interest in him as a child, he was just a stupid boy who at the time I could beat on field day at school in any event LOL. I didnt think about him much.
Then in high school when my current boyfriend Ricky broke up with me for that slut Donna who went to another school. (She put out) I was crushed of course so my girlfriends flanked me in student center trying to cheer me up. They said all the normal loyal girlfriend things …
He is a dirt bag, He will regret it, He’s a pig… Hey we were 15 LOL
So Mia says pick another guy and we will reel him for you. LOL (we work in packs at that age you know) I said I wasn’t interested right now in anyone. It was the end of the day so we stood up and looked over towards the courtyard entrance to start out to the parking lot and there he was walking towards me with Mia’s current boyfriend. He was a year ahead of me in high school but had quit so I hadn’t seen him in a few years and he was just coming in with Brian to pick up Mia. I dont know what happened but in that second without saying a word to each other I fell in love. I got that weird tingly feeling in the pit of my stomach. I froze up though and didnt tell Mia. I probably had the DIHL look. I still remember exactly what he was wearing down to his boots and how white his teeth were when he smiled at us and how he smelled like ivory soap. LOL From that point on we all hung together as friends. He didnt ask me out and both of us saw other people but I knew I loved him. My father wasn’t to fond of him at first even as a friend. I think he sensed it.
Then one day I was between boyfriends and he just broke up with a cheating girl and we were all hanging out consoling him, telling him all the stuff loyal friends tell each other at that age …She’s a slut, Your to good for her, She doesn’t even like the things you do, play the field… bla bla bla and we asked him to describe the ideal girl. He did and Brian starts laughing and says "you realize you just described Tish right ? " He asked me out that night and we got married a year later when I was 19. Everyone said your to young, explore life more bla bla bla…but you know when you know.
He is out working in the yard right now and I went to bring him a drink. He looked up and smiled and that same weird tingly feeling flipped in the pit of my stomach.
So thats my love at first sight story. I think it was for me but not him. He sort of grew into love with me although he says otherwise. Nope no pheromones involved even natural ones. I knew from a good 50 feet away. 
Tisha