Many things, but I guess the fact he (Yar, on the right) kept a look-like of the guy on the left (Andrew) whom he r***ed really keeps me up at night.
Andrew is cursed - any Mortal who looks at him, falls in love with him, which on hand can excuse Yar as it's not his will to fall for him BUT that thing above is too much. Besides, Yar is rather emotion less and right after he had a nice time out with original Andrew (like, really nice, platonically romantic) he would go r* that look-alike and God knows what the hell was in his head.