Humans often succumb to infantile behavior. Your cat has her moments too. And there are lots of them for a domestic cat.
As humans we keep our cats in a perpetual state of kitten hood, until they wander out at night (if you allow that) and instantly become wild cats stalking prey.
There is no need to stalk prey indoors though. If your cat is a lap cat you've probably felt your cat's claws on your legs.
When you sit down you are giving a signal that you are a mummy cat ready to breast feed your cat. Your cat jumps up and being reduced to infantile pleasure kneads your legs/knee in simulating what she would do if she were at the breast of a real mother cat.
In kneading your knee she is stimulating the production of milk. The trouble is it's your knee so not much milk is going to be produced.
Still acting on instinct she presses on and may even salivate all over you in anticipation of the meal to come.
It is your beholden and avowed duty to play along. You should not move or become distressed at the agonizing pin pricks rhythmically being inflicted.
Keep pretending that you're Mum and provide a loving warm environment for your cat at this moment because for your cat it is just that, warm and tender. To throw her off would be almost cruel and difficult to understand for your cat. It may even be hurtful to your cat and send the wrong signal. It may make her insecure as well.
To a cat we are maternal figures so we must act in that way. They are seeking the warmth that a mother provides; we shouldn't disappoint them.
Picture reproduced under creative commons (forgotten credit - sorry)