Dumbledore's Army: Why is Voldemort's face snake-like..?

Maybe it's because when Voldemort was in Albania, he could possess the body of others. He frequently possess the body of snakes. That means, by that time, he already took an interest toward snakes. Maybe it''s because Slytherin is symbolized by snake. Or, maybe it's because he discovered he is Parseltongue. Or, being a "curious boy", maybe he discovered dark magic related with snakes.