Thais regard the head as the highest part of the body both literally and figuratively. As a result, they do not approve of touching anyone on that part of the body, even in a friendly gesture. Similarly, if you watch Thais at a social gathering, you will notice that young people go to considerable lengths to keep their heads lower than those of their eiders, to avoid giving the impression of "looking down" on them. This is not always possible, of course, but it is the effort that counts.