Common man is very much taken up by the idea of a superhero because it provides an alternate reality to an everyday, monotonous life. Superman, Batman, Spiderman, The Flash, Phantom, and Wolverine are only few of the superheroes that Marvel or DC portrays so fantastically.
The idea of good triumphing over evil or the mere fact of possession of such supernatural powers is enough to catch the attention of the general public. These heroes have two sides to them, the superhero side with the ultra cool powers and then the other side, wherein Spiderman is just Peter Parker; an ordinary press photographer.
Bruce Wayne saw tragedy at an early age, which spurred the birth of Batman; a dark knight who comes to the rescue of people who are in need of help. Superman is just Clark Kent, an ordinary journalist who can never be with Lois Lane because of his responsibility and everything that comes with it.
It is noticed that the protagonists of all these films are never shown to be perfect. They are flawed and vulnerable and every bit human, just like the general public. The struggle is real with them, but it all boils down to the choices they make. Every superhero has to give up on the good things in life in order for the larger good. Every superhero is shown to have a strong side and also a vulnerable side that causes him/her to get attacked.
Every one of them is shown struggling with the responsibility that comes with their title. And isn't all of this synonymous with what the common man does in his or her life?
The point of this being - people can relate to their favourite superhero; to the glamorised, glorified side and the ordinary, common side and that's why there exists the fanaticism when it comes to superhero movies.