Doing the grocery with a friend of mine

A friend of mine offered me to take me at the grocery store tonight. I should have seeen it come, we went at the grocery store and once we were there, He asked me if I could buy him a quart of milk, I said yes.

The problem is,most of the time when I go to grocery store with him, He often asks me if I can buy him something like a loaf of bread, milk, or cream for his coffee.

Sometimes that frustrates me, which is probably why when He asks me if I want to go at the grocery store, I simply say no.