My girlfriend was eating some pizza and I had the urge to grab it from her and eat it.
Why would you want to eat her pizza?
She was paying for it so it's natural for me to grab a slice from her order.
That's her pizza not yours, it is wrong to steal a slice of your girlfriend's pizza. You should buy your own pizza.
Why would I buy myself a pizza when I can have it for free in the place where I live?
Because taking your girlfriend pizza is going to upset her, I don't think that's really worth it in the long run. Imagine if she was feeling your food, how would you feel?