Early on in their partnership, Reese seduces Finch because sex is a good way to gain someone's trust and find out their secrets. By the time he starts to genuinely care for Finch, and starts to feel bad for trying to trick Finch into a fake intimacy that way, he realizes he really fucked up: Finch is in love with him, and thinks they're in a real relationship, and the last thing John wants is to hurt him.