To understand, appreciate, and enjoy the vocals behind metal bands that use a singing style that is "growling, screeching, or screaming" you have to acquire the taste for it. It also takes an open-minded route to want to understand that the harsh vocals and singing vocals are the exact same thing. Whenever I first joined the metal scene the only bands I listened to were Iron Maiden, Iced Earth, and Blind Guardian. I got into Cradle of Filth and Dimmu Borgir afterwards, but couldn't get into the heavier styles of music. Nile, hypocrisy, Bathory, black metal and death metal in general, because I didn't "like" the vocals. However, the reason for it was because I was used to the thought process that singing only involved the typical, traditional sound behind it. I wasn't truly giving it a chance in other words.
Plus there is a spectrum of heaviness so to speak. Try getting into the popular acts if you wish, but make sure you're open-minded enough to understand the material at hand. It may sound like an ignorant method of using your voice to good instrumentals...but once you get passed that, metal takes on a whole new level of musical prowess.
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