Cats knead usually because the cat is happy and content.
My cat always kneads his paws when I have him sitting in my lap or laying on my stomach on the couch.
He starts to knead because he's happy and relaxed.
Some cats may also knead when they are bored and the kneading helps get rid of the boredom that the cat is experiencing.
It depends on what the cat is doing as they can knead for different reasons but if the cat is kneading you then they are just usually happy.
But if the cat is just laying around somewhere else and kneading they could be bored or even anxious as well.
Some cats also suckle on the things they are kneading.