In Avengers: Endgame, why was Captain America crying when he saw Thanos dust away? Are the two related?

In Avengers: Endgame, why was Captain America crying when he saw Thanos dust away? Are the two related?

Okay, I have 2 theories, so bear with me.

For the first, I think that the tears that Cap shed during Thanos's dusting weren't in mourning. I think it was much more likely that Cap was overcome with emotion at the thought that they not only restored the missing half of the universe's population, but they had also finally defeated the enemy that had caused the Avengers so much anguish over the years, once and for all. And he was dying in the very same manner he had killed trillions 5 years ago. Cap was definitely beyond elated that the hardest battle they fought ever as a team was finally over, and they were the ones who were victorious.

The other theory I have? He wasn't crying because Thanos turned to ash. But he was beginning to cry over another unfortunate casualty from the Infinity Stones: the man who used them, Tony Stark, aka Iron Man, aka "the Earth's Best Defender", as Cap referred to him in Infinity War while Thunderbolt Ross accused Cap and his covert team. Think about it: before the Time Heist was made possible, as it was still just a theory they were working on, Tony had a wife, who he had helped raise a young girl with, in the events following the worst genocide in the universe's history. And Tony loved his new family more than anything in the world at this point. It was a second chance for him to do right, without being an Avenger. He had even effectively retired from avenging around this time, it seemed, and had a small plot of land with Pepper, the love of his life, the woman who stood with him, even at the expense of her own sanity and well-being, at times. She sacrificed a lot for him, and he was willing to sacrifice all for her and their young daughter to have a better universe. It wasn't until Tony discovered Quantum-powered time travel that he realized it could be done. He genuinely had major reservations about it, though. He was ready to lock up the time travel equations and drop them at the bottom of their lake. But Pepper asked him if he'd be able to sleep after that, knowing that he kept half the universe dead, when he could've done something to change it. That was enough for Tony to change his mind, as the next day he had constructed a device using his nanotechnology, Pym shrinking technology, and even using some Guardians of the Galaxy technology to devise a way to navigate the Quantum Realm and travel to the past to recover the Infinity Stones that were destroyed in their current timeline. He showed the devices to Cap, even going as far to making amends, and reinstated Steve as Captain America, by giving him his shield back, despite Steve being reluctant to have Tony's full trust again. But Tony was also concerned. He confided in Cap that his top priority next to restoring the universe was to keep what he had gained in the wake of the destruction. He wanted to continue being with Pepper, raising their young daughter, Morgan, and enjoying their life together. He didn't want to die trying to fix everything, either.

Regardless, he recorded some final words in the event of his own death, and prepared everything as best he could in the event that he wouldn't make it. He knew he'd be leaving behind so much, but in the end, he knew that Morgan would have a better life, even if he wouldn't be around to see it. And that was enough for him. He eventually stole the Stones from Thanos in the climax of the battle, and snapped his fingers, fully ready to take what came next: his own death and a close victory for his team and the rest of the universe.

Cap had lost friends in World War 2, and he'd been through similar situations before, but he promised Tony that he would do his best to return him home to his young family. And now that man he'd promised had laid his own life down, for not just him, but everyone else, and even the entire universe. Cap was no doubt overjoyed that they had won, but after witnessing the cost victory came with, he grieved. He cried over the loss of one of their leaders, and the friend he had only recently regained trust with.