Why are people so ... serious (???) when it comes to music?

What I mean is like when people say things like , “that music sucks ! go listen to some real music !” and stuff like that. Wouldn’t real music be like Beethoven or Mozart and stuff . I don’t know , but it just really bothers me when people makes such a big deal out of what other peoples taste in music are and start being so rude . It's not like their gonna change that persons mind . I mean I don’t like listening to american rap all that much but I don’t go around saying “rap is crap” .