To me the expression refers to someone holding bad feelings against someone who doesn't know or care. They are "living rent free" in your head because you can't get rid of them, and they aren't "paying in return" because they have no thought of you.
Like one-way rivalry?
Jean Sheppard used to describe the "rivalry" between his white Sox and the dreaded Yankees
" We (white Sox) hated, i mean, HATED the yankees......and the yankees never even knew they were in a fight."
Hate to admit it, but i have to deal with that dilemma at times.....not a good mood to be in.
