It depends on who's kissing you and what it means as processed through your mind. If it means you are being raped by someone you don't know the kiss is pure terror processed through the amygdala! If its the girl you've been dreaming of finally coming round its the pleasure center. If its your ex who you know you shouldn't be with but its just needyness its pleasure center slightly overcoming the prefrontal cortex telling you its a bad idea.