Blind Al: Can you speak up? It’s hard to hear you with that pity dick in your mouth.
Deadpool: Pump the hate brakes, Thanos.
Deadpool: So you're an alien, I guess. How does that help us?
Shatterstar: I'm basically better than you at everything.
Deadpool: Just once, I'm gonna find a planet of people that are worse than me at everything. A bunch of functional idiots. I'm gonna go there, and I'm gonna be their Superman.
Weasel: Isn't that Canada?
Deadpool: You shut your goddamn trash mouth.
Deadpool: Give me your best shot, One-Eyed Willie!
Deadpool: So dark. You sure you're not from the DC universe?
Juggernaut: I'm gonna melt you down and make a cock ring.
Deadpool: Well, that's just lazy writing.
Ryan Reynolds: [Looking at Green Lantern script] You're in the big leagues now, kid. [Shot in the head by Deadpool].
Weasel: It’s time get back on LinkedIn.
Answer: That's one of his superpowers, "Comic Awareness" (sometimes called 4th Wall or Medium Awareness). This is something that Deadpool has in the comics and was incorporated into the films. Other than being a framing tool and to be funny, I don't know if the creator, Fabian Nicieza, or any writers have said why they gave Deadpool that power.
Deadpool's trait of breaking the 4th wall appears to have been the idea of writer Joe Kelly, as Deadpool didn't break the 4th wall for the first time until Deadpool #28 in May 1999, more than 8 years after his debut.