Dark Arts are all about power and ambition, both of which are a Slytherin prerequisite, not to mention that we tend to want that which we cannot have. Snape wanted DADA because he couldn't have it, and also because it's a position of prestige, which would both recognize his mastery of Dark Arts knowledge AND utilize his intimate experience for the betterment of others, thus satisfying some of the guilt he feels about having served the Dark Lord.