My cat always has to have her paw on top!! Why??!

That's cute.  

I am not positive if it is the same with cats but I know with dogs that is a big dominance thing. The alpha always is on the top. I would be surprised if it the same with cats. I know for sure my cats have an alpha who controls the other two. 

I don't know if it's a dominance thing, or a protective thing, or the need to not feel restrained/held down.  I do know that a few of my previous cats were the same and always had to have their paw on top of my hand.   So long as I'm able to cuddle them, I'm ok with wherever they want to keep their paws,  LOL

My cat Ozzy is like that.  He likes to sit on my lap while I'm at the computer and he always puts his paw on top of my hand.  It makes it tougher to type!     But it's so cute--he kind of curls his paw around my hand--that I don't mind.  

When you're putting your hand over his/her paw, you're restricting it. Most cats don't like to be restricted.

Katy does not like me playing with her paws so much, but her tail, she does not mind at all.  When we are sitting on the sofa in the evening, I like to pick up her tail and gently hold/play with it.  Katy does not mind a bit.