How did the words "petting" and "necking" come to mean kissing with passion?

Solution 1:

The verb 'neck' meaning "to kiss, embrace, caress" is first recorded 1825 (implied in necking) in northern England dial., from the noun.

I would imagine the implication is that the activity took place from the neck upwards.

The sense of 'petting' meaning "to stroke" is first found 1818. Slang sense of "kiss and caress" is from 1920 (implied in petting, in F. Scott Fitzgerald).

The common-sense trajectory seems to be the use of the word in relation to domestic animals, then children, then adults affectionately, then romantically.

See 'petting parties' here.

All very tame compared to bussing, it would seem. :)

Solution 2:

I am by no means an expert. However, I do observe and like to make a hypothesis based on the observations. I think it may be possible that the term "necking" may have come the observed actions of many birds' mating rituals. The observation that I made was the mating ritual of the common dove. I noticed that two doves that are established mates occasionally go through an intricate dance that seemed to terminate with both birds pecking and grooming each others neck. Just a thought from a common man.