Tear Jerker / My Hero Academia: The Two Heroes

• While the initial scene with Young Might is rightfully portrayed as awesome and a demonstration of what All Might was like in his prime, there's a very sad feeling underneath it all when you realize just how much All Might truly ended up losing when he suffered that terrible wound. His youth, vibrancy, strength, and overall sense of optimism in his dream is strongly contrasted with the present day, where he's been reduced to a shell of his former self, and now having to struggle just to be able to maintain the facade of the hero he once was, even as his health and power declines. It's something that is noted and emphasized a lot in the main series, but this is arguably the first time we have a clear idea of just how drastic his decline really is.
• The scene where Dave discovers just how low All Might's quirk level has dropped. It plays out very similarly to someone reacting to learning a loved one is dying from a terminal illness. Considering the state of All Might's health with his condition, it's not all that far off.
  Dave: (disbelieving) At this rate, the Symbol of Peace will be lost forever...
• The entire reason why David Shield and Sam orchestrated the attack on I-Island. Years ago, the two developed a helmet designated to maximize the power of one's Quirk. However, the higher ups of the island deemed it too dangerous and ordered it locked up in order to prevent it from reaching the wrong hands. And because of the discovery that All Might's power was fading, he stole the helmet and believed by utilizing it, he can remain a hero.
  Dave: Please, please let me get this to All Might! THERE'S NO TIME TO REMAKE IT!!! ONCE HE HAS IT, I DON'T CARE WHAT PUNISHMENT I RECEIVE AFTER!!
  • What's even worse is that Sam later betrays him to Wolfram in order to have him sell the helmet, for the fame and glory.
• Melissa's reaction during the entire revelation. God it's heart-wrenching. Perhaps the worst part is when she rips off the bandage on her arm, revealing the injury that she got protecting Deku, while saying that Deku and the others are risking their lives to save everyone on I-Island. David is forced to realize that his actions endangered many people, including his daughter.
• Deku's scream of despair after failing to save Shield.
  • The likely reason why he was so affected is that he and Melissa share very similar backgrounds. They both grew up quirkless with only one parent they could really rely on being present in their lives. Deku was probably empathizing with Melissa about how he’d feel if he were to lose his own mother the way Melissa was seemingly going to lose Dave.
• While David did everything out of good intentions and was ultimately tricked by Sam and Wolfram, and was ready to face punishment from the beginning, it doesn't change that his actions endangered thousands of people and almost caused a dangerous technology to be unleashed on the world. Melissa has a bright future ahead, as a scientist potentially even better than him... but with a father now behind bars. Unsurprisingly, the last shot of the movie is of All Might and Deku looking at I-Island not triumphantly, but somberly.
  • On top of that, the strain All Might went through to first maintain his muscle form longer than he predicted, and then fight against Wolfram later, have probably further consumed the remaining embers of One For All inside him; ultimately David, who wanted to help his friend, may have sadly contributed in accelerating the ultimate loss of his power.
• This film's climax was the first time All Might and Deku fought side-by-side with eachother, and -baring the possibility of non-canon future movies- it will be the last as well.