Well, some people just don't like the sound of electric guitars, period. Others are put off by the fact that metal usually requires the listener to pay to attention in order to enjoy it and they don't listen to music with that mindset, they just want something silly and upbeat. Often, the hatred is born out of ignorance. Most people who claim to hate metal do not know that the word is just a label that includes a multitude of different subgenres, each of which sounds different enough that it could belong to a different category of music altogether. Getting to know all those subgenres and discovering the ones that you enjoy the most is also something that requires a lot of time and commitment, which is not what some people are prepared to offer.
As for me, I like many different kinds of music, from weird experimental electro music to Krautrock to silly but fun bands of today, like Black Eyed Peas. As for metal, I am 28 years old and have been listening to it since I was 13, without ever losing my affection to it. Discovering Iron Maiden when I was 13 was a great moment in my life, and I never stopped finding new metal bands that I love since then. I never saw it as anything too special though, I never felt like I "belonged" somewhere because I like metal, nor do I listen exclusively to it. It's just music and I love it, others don't and that's fine. I'll never say no to listening to "Dead Heart In A Dead World" album by Nevermore again however, even if it's the thousandth!