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If you haven't checked out this article about the new album mastering paradigm, you should. The long story short is that some people at labels have decided that dynamic range is a thing of the past, and that people will only notice songs that are louder and denser and--not quite logically--harder to listen to than any other songs. It'll answer a lot of your questions re: why listening to pop radio leaves you mentally and physically fatigued. (The other answers are: autotune, and too many people thinking they're Max Martin when they're not.)
If you need a concrete example of what they're talking about, listen to Vanessa Hudgens' new album V--and specifically its lead single "Come Back to Me"--through headphones. It hurts. Every element of the song is equally, blaringly loud as every other element. Trying to pay attention to any single part is like trying to pick out the path of one ping pong ball out of a truckload of them being dumped on your head. I can get through the intro, one verse, and one chorus before the headache kicks in. But it's strangely great on the same level. The aural pain it delivers is sort of like blasting Reign in Blood until the speakers break, and sort of like the kind of back rub some people give -- the kind where you'd be begging them for mercy if you didn't know that the pain is part of what makes it work.