Cats are very scent-orientated and they rely on scent marks to recognise territory. Rubbing themselves against people is part of normal cat greeting behaviour, and it is a method of exchanging scents with "friends".
Corners of the mouth
Temples of the head
The scent marking that cats perform is not limited to "friends" , but extends to inanimate objects as well such as furniture, garden fences, trees and so on. The rubbing (and also urine spraying) in the environment helps the cat to recognise it's own territory, and also other cat scent markings help to warn when it is entering a strange cats territory. - (Health24, May 2011)