It's because when he's being honest, Daken is empty inside. Actually, both of them are. Laura is trying hard to better herself and fill her emotional core with other things to draw strength and joy from other than the abuse and pain of others. Daken "retreats" to that state when anything comes close to getting through his emotional armor, clawing away at anything that dares get close to him. Only when he seems to be using others does he pretend to be vulnerable.