Being a lover of movies, I sometimes watch a movie that I find very inspiring, motivating or just aesthetically pleasing and I sometimes that many people like it too. However, it is not uncommon to find criticisms from film critics who conclude that the movie is trash or below standards and are not worth watching. My question is: who decides if the movie is "really" good and worth watching: the film experts who don't like it or the public who adored it?

After a hard day's philosophising, I find nothing more relaxing/distracting than a very loud, stupid movie with lots of things blowing up. The kinds of movies that, with rare exceptions, critics hate. Which of us is right? Well, neither, AT LEAST because we are applying different criteria. I want something mindless; critics rarely do, because genuinely intelligent or subtle films are what they want after a long day's watching loud stupid films.
Now, let's take your inspiring movie. Again, its quality of being inspiring to you, and to many others, is only one thing a movie might want to do. And some such films work very well and fare well also with the critics -- like the first Rocky, for example. But a film critic has to balance a lot of different possible criteria: story-telling, script, acting, direction, etc. He or she then has to give thumbs up, or down, based upon this balance. That is why film critics often write things like this: 'Well, if you liked Rocky, then go see this; but otherwise...'.
What I would do is read several critics for each movie you watch. With luck, a pattern will emerge, in that critic C will like the same movies you do. Then, you've got a critical guide, with whom your tastes are aligned.

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