I feel like the relationship between music and expressing pain is a tough one to boil down. Going by what I get from pop culture nowadays, a lot of the modern artists I feel exploit the idea of "the suffering artist" in order to amass a larger fan base and sell more albums. In some ways, one might say, you have to respect that as well. The artists that do this to make a buck are enough in tune with the pain and suffering some people can go through in order to write a song that everyone can feel related to, so that has to count for something, right?
Not so much, in my personal opinion. Nowadays, in our culture, I would argue that more people know of various types of physical and emotional pain rather than actually feel that way, thanks to various pop culture movements and the art of making cookie cutter movies. In my opinion, it is this knowledge, followed by the "fear of pain" that causes us to relate to the first embodiment of that pain we actually see, and it is that knowledge of how we are supposed to react to pain (thanks to songs, movies, etc) that then dictates how we actually react. Which then, in turn, leads us to buy more albums from that one artist that just totally gets it. This self perpetuating cycle is seemingly endless, and kind of depressing if it is true. But while I am cynical, I also can think of it from the other point of view.
On the flipside, there is no denying that there are artists that use music as a mode of self expression. Hell, they ALL might even do that, but I don't entirely buy that there are no artists out there faking pain to make a buck. Back to the point, though; some people, like Kurt Cobain from Nirvana, were generally messed up. Music then most likely became a way of letting those few people put their experiences and feelings into words; in a way, like the transition from the Chaos Narrative to the Quest Narrative. These songs are essentially the artist's journey through a painful experience, and when they are all put together they seem to provide a self-medicated storyline that almost anyone can listen and connect to.
Clearly the situation changes with the artist, song, and topic, but I feel in general that it is one way or the other.