tl;dr
How much do you think music reinforces or influences your state of mind? If you're depressed and have a generally negative outlook on life, the universe and everything, is metal and 'new rock'/alternative (let's not even get into emo) going to help keep you there, even if you feel like you're having fun while you listen to it? What about pop music: will it keep you in a lustful or infatuation prone state? Will hip-hop keep you arrogant and quick to anger?
/tl;dr

I would say yes, the music you listen to reflects and reinforces your overall state of mind to a significant degree, particularly if you're like me and listen to music several hours each day.

I was reflecting today on how my musical tastes have changed in the ten years since my teens and early twenties. Most of the music I loved and that really meant something to me back then, if it even gets a spot on my iPod now, gets skipped more often than not when it comes up on shuffle. Some of the stuff I liked back then would be Pearl Jam, NIN, Afghan Whigs, Counting Crows, Catherine Wheel...mostly dark, angsty, depressive and/or angry stuff, which pretty well covers my mindset at the time. Once in a while that stuff will still click with me or I'll give it a nostalgia spin, but my usual response is: nope, too negative--*skip*

These days I tend to get into more complex, more abstract, and more upbeat/celebratory music: jam bands, jazz, 'sparkly' electronic. The outlier would be blues, bluesy folk and strongly blues-influenced rock, which keeps growing on me despite being pretty much the definition of a downer

Now, I also know that when I am down or surly, the really upbeat stuff in my collection will make me want to to throw up or hit someone I'm not saying you can force yourself to be in the mood for something if you're not, but maybe just mixing it up a bit, bringing in a few more upbeat songs or just something abstract and inoffensive can be therapeutic. I'd liken it to the Tone Scale (yes it's from Scientology, but I think it still breaks down mood/energy level pretty well):
Spoiler for Tone Scale:

You're not going to jump ten steps up the scale by listening to a pretty song, but maybe you can ease your way up.

What do you think?