Explanation
In the United States, patting someone on the head is typically reserved for children or intimate relationships and can be perceived as condescending or a violation of personal space when done to an adult, especially in a social gathering. It may also be seen as patronizing or infantilizing, as the head is considered a personal and sensitive area. Exceptions exist for close friends or family in playful contexts, but it is not a standard polite gesture.
Local Tip
Avoid patting anyone on the head unless you have a very close relationship and are certain they would welcome it; instead, offer a handshake or a friendly nod.
How it compares elsewhere
The head is considered the most sacred part of the body, and touching it is highly disrespectful.
Patting a child's head is often affectionate, but doing so to an adult, especially an elder, is disrespectful.
Physical contact is minimal in public, and patting someone's head is seen as overly familiar or condescending.