Basically the way someone will break up with you if they are trying put it nicely (see euphemism). The word "just" is the word that hurts. It's like saying, "Let's never go further than being friends. Never." Take out the "just" and it doesn't seem so bad, right?
"Hey Paul, I really like you, let's just be friends." Ehh, this doesn't work.. There has to be a "but" in there.
"Hey Paul, I really like you, but let's just be friends." Perrrfect. That one hurts more than a freight train.