To be entirely honest, I never really cared for Pokeshipping, or any other shipping (I was always the type to lurk forums and laugh at the shipping wars).
However, I guess I fell into this mentality as I was playing HG and I found Misty at Cerulean Cape with ANOTHER GUY who was obviously not Red (and before I comment any further, I know that Ash is based upon Red and they are not the same thing but in my opinion and my opinion only, the differences don't really matter to me). So I decided to look up who this piece of nameless blasphemy was. And then it turns out that I realized, hey, Ash and Misty really do look good as a couple (I never did find out who the NPC was). And here I am, becoming what I once laughed at. Ah, irony.
And generally, Misty appealed to me most out of all the main female cast because she was the first one introduced to my generation, back when we were still in elementary/middle school. The others just don't have that something that I can't really describe that she does with Ash.
Nope. Love at first sight would be like Ash saying to Misty in episode 1, "Oh wow, I'm gonna be torn apart by a flock of pissed Spearow but it's totally worth it cause you'll be the last thing I see besides my blood/guts and a bunch of beaks and talons".
I see it more of a gradual thing. People have pointed out the differences, such as the families of the two not really having any conflicts with each other and whatnot. I'd tack on more reasons but I'm sure they've been all stated already.