I don’t believe in love at first sight, simply because the word ‘love’ is thrown around with too much irreverence for what it actually is. You cannot love someone until you know what they are like in ways that are not immediately obvious.
That said, I was very interested in my girlfriend from the moment we met. There was something in her smile the moment we were introduced that instantly made me think we had some sort of connection. That smile still gets me every time. Apparently my eyes did the same for her. It was not lust, that came a little later on, it was just an irrepressible desire to get to know her better and spend a lot of time with her.
From a morbidly boring scientific point of view, there are various chemical compatibility tests that we use to find potential partners, although these are only factors that modify probabilities rather than deterministic factors. The tests are evolutionary mechanisms to match compatible genetics and immune systems to help produce strong and healthy offspring. These chemical processes generally act fairly quickly, and may lead to an instant initial attraction whether or not the parties involved pursue a relationship.