Because they are like fractions,they must be the same in order to add them. It's like when you're adding 4x to 7x,you get 11x. But,if you were to try to add 4x to 7y,you wouldn't be able to. It would stay at 4x + 7y.
What I'm saying is,your denominator is what keeps it like. The only thing that matters after that is the numerators you're adding.
