The first reason, the plot. When we usually watch a movie, we are often moved by experiences similar to our own. This is the case with the plot of "Forrest Gump", where we live a rough life, get mocked at work, and live on a shoestring, but still live a wonderful life, forming a ****ing chorus with Forrest Gump in the movie.
It takes a lot of courage for Forrest Gump to admit his stupidity in the midst of life. Just as it does for us to admit the difficulties in our lives. So when we go back and watch the movie Forrest Gump over and over again, we can relate to the movie and learn some qualities that we don't have.
The second reason, acting. Movies focus most on the acting skills of the actors, and most of the good or bad of a movie is in the hands of the actors. Forrest Gump, in addition to its own appealing plot, there is our acting Tom Hanks, the actor, known as one of the most influential movie stars in Hollywood today, and the reason for the popularity of Forrest Gump can not be separated from him.