but what is it about the physical act that's so OMG?

It's a chemical reaction, I guess, so (as I recall, and ftr, it's been over a decade since I kissed someone for the first time) a little like being a little drunk, some head rush, and your breathing speeds up, which can make you a little light-headed, and there's a rush of warmth and (sometimes) arousal, which is alluring and exhilarating (hence the rollercoaster analogy) and feels good. Plus you're entering someone's personal space, face to face, often someone you really really like and want to like you. Hmmm, maybe it's akin to the warmth of being admired by someone you admire, multiplied by lots? With some fizzy-exciting chemicals thrown in?