When Ed and Eddy fake their own death via quicksand while Edd watches and breaks down. Eddy and Edd have a fight about it and Edd walks out, saying that he would rather get his ass kicked by the kids than to wander off with "a so called friend." When Eddy sees that Edd is serious about leaving, he breaks down and cries, admitting that everything was his fault. Edd is so moved by his apology that he decides to come back.
The part of this scene where Edd states that Ed and Eddy are the only two people he cares about gets this troper every time, especially when he considers how distant his parents are made out to be in-universe. The fact that he's given up on his parents ever being there for him makes the eyes water.
Also, when Edd decided to leave Eddy, Ed's words are also tear jerking:
The moment is even more heart wrenching when we remember just how much Kevin hated Eddy and feared Eddy's brother, and now he's sticking up to his fear on the behalf of his mortal enemy.
The reason why Eddy is such a cunt. He wanted to be liked by everyone by making up stuff about his brother and hiding his true, vicious nature.
Eddy: I made it all up, Double-D. Everything about my brother was a lie. I just made things up, so everyone would like me, think I was cool. But boy was I wrong. The scam, my brother... when am I gonna learn, Double-D?
Just seeing what an Extreme Doormat Eddy becomes when his brother abuses him. The most confident, headstrong of the Eds can only whimper and brokenly smile as he begs his brother to stop. And this movie reveals that this abuse had been going on this kid's entire life.
Edd writing the letter at the beginning. It's much quieter than most scenes in the show.
As he starts trying to describe exactly what happened, Edd suddenly starts using adjectives such as "immoral," "thoughtless," and "hurtful," getting angrier at each word. This could be showing how unbelievably guilty Edd feels about the whole thing...or it might be Edd finally losing his temper with his barely-there parents.