Starting out with Warwick - Alternate title: why do I never lose?

Warwick never appealed to me, as I was pretty much caster only, but then I started playing Amumu + Kayle, and decided to give him a shot last week when he was free.

The reason I make this thread is because I have no idea why this build works, and why I'm destroying teams so quickly without any hassle (even in solo que / ranked solo que).

My runes are Arm pen red, attack speed yellow, cooldown reduc blue, health quints. Spells are Smite / Ghost (sometimes flash in normal games for trolololol).

I start the game at blue, clearing the jungle then getting a dragon at level 4 (2 health pots, one smite), then getting madred's razor. After the usual jungling phase (which I don't leave the jungle until I have madred's bloodrazor unless it's for a gank) I rush an infinity edge, then the game ends.

So for the past five games I've played WW, I've ended up with Madred's, Zerkers (or mercs), and an infinity edge, before the game has been won.

These are two high end items that it seems like I shouldn't be getting so soon, but every game no matter what I demolish their team without any problems. Should I be doing something else, or just continuing with starting this way?

Also; for items after these I'm pretty sure damage is taken care of so I would go Wit's End + phantom dancer, but this is all so expensive I'll probably never finish my build in a game.