The film is full of grief, gloomy themes but also light humor, the whole thing becomes really conscious when we see at which points heroes like Thor, Bruce Banner and Tony Stark are standing. Their stories couldn't have been any different. Banner, who had been in conflict with Hulk for the whole MCU, has finally found peace with his two personalities and can now use the best of both.
Well, there are several reasons why we could see a mixture of Bruce Banner and Hulk aka Professor Hulk. In Thor's last movie the whole thing was announced when Hulk could speak there. And after Hulk was beaten worst by Thanos at the beginning of Infinity War, he had withdrawn and Banner had problems to cause to turn into Hulk. In addition, Hulk has become very old and therefore wise, but that's another topic that I will perhaps address again later.
That Bruce Banner would get the power over Hulk was foreseeable, because in the second unofficial Hulk film, which was not done by Disney and therefore the Marvel Studios themselves but unofficially still belongs to the MCU, we can see how Bruce Banner starts to meditate to control Hulk. In the very first Avengers movie Bruce Banner also makes a remark that he can control Hulk. He probably continued that after Infinity War, his meditation training to control Hulk. And from here it also becomes a fan service, because in the comics Professor Hulk is a part of the story and sooner or later he should have been introduced. That it happened in Endgame was a very good time.
Tony has actually overcome his trauma of 2012, where the Aliens invase in the first Avengers film was, through the immense loss. As bizarre as it is, the snap was for Tony's character like a plaster that was quickly torn off because for six years he was obsessed with protecting the world from the threat of Thanos. Now after the snap this pressure is gone.
Well, not really. Tony Stark was a narcissist. At least that's how we got to know him and followed his development in all the films. He had made it to his business to protect the world and his beloved Pepper. He gave everything for this job. He even developed the killer AI Ultron. That was all for the protection of the planet. But Tony had his obligations to his Avengers teammates as well as to Pepper. All his actions were always rational and logical, although not always understood by everyone. He always wanted the best for everyone, even if his methods were somewhat incomprehensible and selfish.
Now he finally has his family, which he had always promised Pepper. Never be Iron-Man again, start a family, have children and lead a peaceful life. He got all that through the snap. The snap was a blessing for him as well as a curse. A blessing that his dream of a family came true and a curse that he had left the world in the lurch. The loss of his teammates gnaws at him, although he doesn't want to show it.
Tony Stark is a rational-thinking character. It is said, if you give the feelings the upper hand, you can't think clearly anymore. So you have to be cool and suppress feelings so that you always have the control over your mind and so you can think clear. That's how we got to know Tony Stark and he does that all the time. That makes him somehow to an asshole, but if you know these things, you see him with other eyes.
Part 2 
Part 4 
