I kiss both my kids on the lips and never really thought twice about it. I will also kiss my mum, dad, nan and pop on the lips but the rest of my family get cheek kisses. I've never really thought about why this is but its just the way it has always been. My husband on the other hand doesn't kiss any one but me on the lips, including our kids. He was brought up that you only kiss on the cheek so does what he is comfortable with.