Hithere ~ I think it is cultural, not instinctive.
On the contrary, showing affection, through touch, etc., is not a cultural, or learned behaviour. In fact, it is learned behaviour to repress our natural innate ability to express affection, and receive it ...
Human beings are a social species who require a certain degree of contact with other human beings. It is a natural human instinct to interact/show affection.
As I said previously, I don't condone randomly going up to a stranger and taking the hand/stroking their child. That would be rude, and impolite, and I think the majority of people would agree. However, when getting into conversation with someone, in a queue/shop/public transport, and their child interacts with you, it is a natural human instinct to give a polite little stroke of the cheek/head, or a little tickle of their feet/fingers.
However, it seems that natural human instincts are being repressed by some, which I personally don't see as a good thing, but each to their own ...
I can only imagine that, in years to come, the human race is going to be rather robotic to say the least.
So glad that I, and my own children, grew up in a time when people weren't afraid to follow their natural human instincts. My little grandchild gets lots of attention when we're out, and thankfully my DS & DiL don't get offended by it. Obviously, if someone was to stick their finger in her mouth, that would be unacceptable, but thankfully that hasn't happened, it's just people being friendly and polite.