He took a purifying potion so he could 'kiss' Juliet awake. A spin on the "kiss from from a true love" thing. Problem is the kiss-potion thing is actually turning into an obsessive love thing. He is obsessed with her and is trying to find an antidote. It also affected Juliet and he can't get a true cure until he involves her with it, which means coming clean to her about everything. Also, he went to Monroe for the antidote and Monroe thought he recognized him but wasn't sure.