I 愛 his completely nonchalant, self absorbed attitude plus his moments of emotion (i.e. when the orphans give him their drawing and he pretends to crumple it up).
I 愛 Bender because of his personality and his hilarious catchphrases. My お気に入り is, of course, "Bite my shiny metal ass." And I 愛 his face when he gets mad. It's priceless.