People share all manner of tastes regarding media, and all are valid. You have expressed that you prefer darker and more "serious" works, which is fine. As a few people have pointed out, Hello Dolly is not as simple as many assume (I especially appreciate Stage Door Sally's analysis of it). However, even if you do take it as light fluff, there are indeed people who prefer that.
I personally enjoy my media to be more of a lighter and happier fare, and especially to have a happy ending. There's already enough darkness in the real world - I don't need more of that in my entertainment. With that said, I certainly appreciate darker, more serious, and more "intellectual" works, and there are plenty I enjoy, but when given the choice between two works, I'll most times choose the more upbeat work.
I sadly have never seen Hello Dolly on stage before (only recently got into this particular medium), and so I've only seen the movie version with Streisand recently, and enjoyed it. I have been told the stage version is superior to the movie, so I'm looking forward to seeing the stage version with Bette in May. I do love the music especially though, even before I actually saw it - my major exposure to its music (as well as that from Oklahoma and The Music Man) was in the background music to Main Street USA in Disney World - I love that music, and it eventually lead me to seek out the actual origins for the original vocal versions.