Because "love is the eternal theme of literature".
But what is the basis of this sentence? Here, I make an analysis based on the data of "Detaiwen 20 19-2020 TV drama market analysis report (III)" (the original Detaiwen film and television observation was released in wb).
A very important factor for TV series to be popular is that the audience can understand and feel it. But the audience sitting in front of the TV may come from all walks of life and have different hobbies, so a TV series wants to be popular, either for some specific people or for the audience's sexual orientation.
So psychologically speaking, people are most likely to feel the emotional direction of * * * is nothing more than these-
Feelings of home and country. For example: tribute drama, historical drama, realistic theme to screenwriter.
This kind of TV play or film, if filmed well, can last for a long time, but it often tests the script and even the original. I still remember crying like a messenger when I saw the Beiyang Navy completely annihilated on the screen. Tv dramas, such as the song dynasty, are always recommended, so I won't introduce them much. The same is true of some classic anti-war dramas. The recent "Action to Break the Ice" is also in this category (but there are still emotional lines in it).
However, according to the data, there is still a certain age limit for this kind of drama according to different themes.