One of the main reasons a dog may want to lay on its owner is that she simply wants to spend quality time with her master. The presence of her owner makes her happy, especially when her owner is gone for most of the day.
Another common reason a dog may want to lay on a person is that she might be cold, and cuddling with her owner keeps her warm. Finally, some dogs are genetically disposed to want to constantly lay on top of their owners. Poodles, Shih Tsus, Lhasa Apsos, Maltese, Pomeranians and Caviler King Charles spaniels are some of the breeds most likely to be predisposed to lay with their owners and cuddle.