The answer is that the neck is a vulnerable part of the body when attacked. I am talking here of primitive times. An animal, when going for the kill will go for the neck of its quarry. When animals are getting to know each other, the submissive one will often roll over to signal that they are just that. In other words they are saying, "Here is my neck you can kill me if you want to but I know you will not."
It is the same with humans. It is a primordial emotion signaling submission. I think the ticklish feeling is there to reinforce the game play.
Leaving aside the above and getting up to date, it is lovely for a man to kiss a girl's neck. It is soft and warm and fragrant and smooth and the girl's hair makes it even more lovely. It is nice to know that it has a good effect on the girl too, if she likes the boy.