'Tis alcohol.

Well, in my case anyway.

To me kissing feels like the animal side of our mind at work. From the small comfort of a gentle peck to the enlivening thrill of trying to snogg someone's face off it is an instinctual communication. Animals will communicate affection for each other with their mouths, humans will too. Animals just don't worry about what it means. I don't think we can put it into a rational context because it doesn't come from the rational part of who we are.

It's been noted that it 'feels good', that's why we do it.

The human lip is full of nerve endings and very sensitive. Kissing stimulates those nerves. So are your hands and especially fingertips - ergo handholding. Human genetalia also has a lot of nerves endings but obviously social/cultural precepts prevent us from bumping our jone's together at-will on the street. The kiss or handhold is the faster and more socially accepted quick-fix.

THAC0 has summed it up pretty well there. It's basically about the stimulation of erogenous zones. Touch, taste, smell are all factors, and the mouth is easily the best place to experience these senses. You get much the same sensations by applying your mouth to other body parts, but mouth to mouth reciprocates most efficiently.

Along with the physicality described above, there are also social and mental aspects which shouldn't be overlooked. A kiss in Western cultures implies intimacy, and the anticipation leading up to a first kiss is more significant than the simple, physical act.

Fixed!

That what Im talking about. Is the desire and need to kiss someone actually an instinct? Or is it a social desire conditioned into us through culture?

Probably both and a few other things besides. Our social behaviors are the accumulation of thousands of years of influences that shape what we are today. Trying to find the one "it" that causes it will leave out a host of factors.

http://en.wikipedia.org/wiki/Kiss

Hand holding also varies greatly across cultures.