It feels like a mostly-random choice of body parts to me!

Well, humans have most of their nerve endings in their hands, mouths and genitals, so I guess that's the main reason, and since we'll put our hands pretty much anywhere (*g*), mouths and genitals are generally considered more private, and therefore more intimate. (Also there are theories about oral fixations dating back to childhood and breastfeeding, but I don't have strong opinions on those one way or the other.) Generally speaking, sexual people are wired to enjoy (and get turned on from) kissing. [Insert usual disclaimer wrt generalisations; there are apparently some cultures who don't kiss.]

Does that help?