Why I drink is a phrase derived from the animation Ramshackle by Zeddyzi, specifically a commonline for the character Stone. The phrase why I drink or This is why I drink can be used to denote extreme agitation or annoyance.
Person 1: does some dumb shit Person 2: *points over* Why I drink
Person 2 (alternate timeline) : This is why I drink
In the closing scene of There Will Be Blood, Daniel Day-Lewis says this to Paul Dano to show that he has defeated thee.
A line that would not nearly be as memorable if it weren't for its pitch-perfect delivery.
"As a sports metaphor? (“Let’s face it. The Celtics drank the Knicks’ milkshake last night.”) An amorphously obscene double entendre, hearkening back to its Kelis-ian roots? (“I’d like to drink your milkshake!”) Or maybe, in a nod to the godlike venom of its utterer, a taunt: “You best back down before I drink your milkshake, bitch.”
--nymag.com