Why does he do that?

Libby and I have a cat, a wonderful cat who goes by the name of Keziah.
But Keziah is like all cats. She is demanding and self-centred, greedy and sometimes destructive. She is always after something. We get up in the morning and she is immediately asking for food. After she has eaten, she wants to be on my knee and for me to start petting her. Then she wants out. Then she wants in. Then she wants fed. Then she wants petted. Then she wants out. And so it goes on. She has even learned to beg like a dog for food. She is always after something, and if we ignore her she might resort to picking the carpet to bits with her claws.
Why do we do it?