Warmth
He emanates heat, fully enveloping and inviting; a mere taste of the warm and amiable disposition lying underneath. He is full of love and friendship, and more than willing to share it; spread of fire. But, like most things, that fire falters and sometimes outright goes out, but he never loses that warmth.
He, on the contrary, lacks that heat; rather a cold, empty shell of what he could—should—have been. He is a fighter, driven solely by the will to survive; love, compassion, and generosity are just foreign concepts to him.
He tries to avoid the heat, tries to push away all that he never had, never understood, and—what he believes—never will. But eventually, the heat gets closer, slowly melting away the frigid surface of him. And, for awhile, he allows him to get closer, to finally feel the warmth he's been so deprived of. However, forever wary of anyone's intentions, he ultimately pushes him away, his cold demeanor getting the better of him; freezing the lock shut.
Because with heat you're bound to get burned.