i think it is somewhat instincual. think about how long humans have been "kissing"
Also, what i noticed is this: I have two dogs, male and female. The male (buddy) starts to kiss the female (kiva), and by kissing i mean licking her face intensly. Then he starts showing signs of arousal and often tries to mount her (although, she does'nt let him). The licking seems to be like foreplay. I just found that interesting how it seems to be a sexual thing in dogs, as with humans.