Happiness for some people may be a warm puppy, but others swear by a sleeping cat. There's some controversy about people and pets sharing a bed, but it's hard to keep a cat out of your bed -- except by his own choice.
Love
The simplest explanation is that he loves you. Many cats undoubtedly reciprocate the affection their human partners feel for them, and one of the ways they show it is by cuddling.
Physical Comfort
Cats like soft, warm places. You are warm and your bed is soft, therefore he likes to sleep close to or even on you.
Togetherness
Many cats like sleeping together. It may be they remember being kittens, piled in a warm, fuzzy heap next to their mother's bountiful belly. They share warmth and affection with each other, and in the absence of other cats -- or even in preference to them -- you make a good substitute for a bundling buddy.
Mental Comfort
If your cat is nervous or anxious about something, sleeping next to you can make him feel safe. Incidentally, this can work the other way around, too. There is no lullaby quite like the purring of a cat -- the ultimate in white noise.
Ownership
It's the cat's bed and he magnanimously allows you to share it. After all, you haven't rousted him out to establish your rights. He's marked both you and the bed with his scent. He decides when he sleeps and when you do, and he decides when you wake up to feed him. In his mind, that makes him top cat. Remember, dogs have masters -- cats have staff.