Wanda Maximoff: Vision, are you not letting me leave?
Vision: It is a question of safety.
Wanda Maximoff: I can protect myself.
Vision: Not yours. Mr. Stark would like to avoid the possibility of another public incident. Until the Accords are a more secured foundation.
Wanda Maximoff: And what do you want?
Vision: For people to see you...as I do.
Clint Barton: The futurist is here gentlemen! He sees all. He knows what's best for you, whether you like it or not.
Steve Rogers: I know we're not perfect, but the safest hands are still our own.
Steve Rogers: This job... We try to save as many people as we can. Sometimes that doesn't mean everybody, but you don't give up.
Tony Stark: Captain? You seem a little defensive.
Steve Rogers: Well, it's been a long day.
Tony Stark: If we can't accept limitations, we're no better than the bad guys.
Steve Rogers: That's not the way I see it.
General Thaddeus 'Thunderbolt' Ross: Captain, while a great many people see you as a hero, there are some who prefer the word vigilante. You've operated with unlimited power and no supervision. That's something the world can no longer tolerate.
Tony Stark: All right, I've run out of patience. Underoos!
[Webbing flies in, stealing Captain America's shield and restraining his hands. Spider-Man lands on a vehicle, holding Cap's shield.]
Spider-Man: Hey, everyone.
Steve Rogers: I'm sorry, Tony. If I see a situation pointed south, I can't ignore it. Sometimes I wish I could.
Tony Stark: No, you don't. Sometimes I wanna punch you in your perfect teeth.
Scott Lang: I want to say I know you know a lot of super people, so thinks for thanking of me... Thanks for thinking of me.
Steve Rogers: Sorry, Tony. You know I wouldn't do this if I had any another choice. But he's my friend.
Tony Stark: So was I.
Chosen answer: Because the U.N. has been preparing to pass the Sokovia Accords and they want to oversee the Avengers. A week earlier, the Avengers would not have had to seek permission to find the other Winter Soldiers (or wait while the oversight committee makes a decision). As a result of the U.N. Sanctions, Captain America and Sam Wilson have to go rogue in order to find the other soldiers. (Their actions of going rogue result in Iron Man and his team in hunting the rogue Avengers down, creating a "civil war").
Bishop73