Why do people bash movies? I just saw a movie that was great and cause it wasnt number 1 people scream FLOP!

Why do people do that-instead of seeing how the box office turns out at the end they immediately scream flop and bash the movie.?? Jersey Boys which was VERY GOOD- won 4th place on a 40 million budget. Its just hurtful to see a movie tu like get bashed especially when it was a great movie.