While not all pets have the same preferences for physical contact, cats generally enjoy it when you pet them on the head. This is a combination of both physical and emotional factors -- for example, your cat may associate head-petting with pleasant memories, or maybe he just loves the way it feels. In any case, your cat has a reason he enjoys the experience, so you may as well indulge him.
Scent Gland Socializing
Cats love marking their territory, and when you stroke yours on the head, you do some of the work for him. Your cat has scent glands around his cheeks and temple -- you may have noticed that he uses these parts of his face when he rubs against the couch, the blankets, your pillows and virtually every other surface in the house. The scent may be too subtle for human noses to detect, but when you rub his head, you pick up his own unique aroma, and he doesn't mind that you're taking the lead for once.
Like dogs, cats can learn from repetition, and that includes growing accustomed to a certain petting schedule. For example, if you habitually pet your cat on the head while sitting in a certain spot, like the couch, or performing a certain activity, like reading, he may develop a habitual love of the practice. Some cats grow so fond of their routines that they love being petted on the head, but only under the circumstances to which they are accustomed -- if he enjoys it in one area of the house or at one time of the day, but not others, he may just be a creature of habit.
When a cat is a kitten, his mother habitually grooms him with her coarse tongue, cleaning his face, ears and the rest of his body. He doesn't necessarily expect you to give him a facial massage with your tongue, but petting him is the next best thing. When you pet your cat, the sensation is a close approximation of the grooming process his mother lovingly subjected him to as a kitten. It helps him relax, and he interprets it as a sign of love and care.
Don't Push It
Even if your cat loves being petted on the head, his love knows a few bounds, and you should always respect them. Petting aggression is a common behavior in cats that they exhibit once they've reached their limit -- according to your pet, there really is such thing as too much of something good. Because your cat's signs of irritation may be difficult to distinguish from signs of enjoyment (tail swishing, for example, indicates either one), you may catch a quick nip or a scratch from a cat who loves having his head rubbed, but has reached his limit nonetheless.