Major SPOILER:
In Avengers: Endgame all the characters live in a very deep state of depression for years. Black Widow, Tony Stark, Steve Rogers, Hawkeye, Thor, etc ... All except Bruce Banner.
In The Incredible Hulk (2008) we are shown that Banner can not have sex because when his heart rate accelerates he can transform into a Hulk. In Avengers: Age of Ultron (2015) Banner tells Black Widow that he can not procreate children, alluding to the theme that he can not have sex.
In Endgame Bruce Banner and Hulk achieved the perfect balance to become a single person in body and mind, and all the time we see him as Professor Hulk, he is super happy in life, making jokes and very relaxed completely. It completely goes against the rest of the Avengers.
Why is he so happy? Simple: Bruce Banner can now fuck again.
During that period of 5 years, Banner has gain full control of his body and has been "crushing", taking off all the fever that he kept for years.
So that's why Hulk is so happy now.
If you have a friend who only lives in bitterness, you already know what they are lacking in their life.