"Sit up straight."
"Be proud, stand tall!"
How many times as a kid have we heard about our posture? I know I rolled my eyes every time my mother would make mention of my hunched shoulders. However, now that I have taken numerous anatomy classes (and aged a few years, which allowed me to experience the fall out of bad posture) I can say that your posture does very much impact your health.
Human beings stand on two legs. It is a neat little trick that allowed us to overcome several early disadvantages. But standing on two legs, rather than four, requires a huge number of muscles to maintain. These muscles work to keep you upright during all the various activities of the day. If you have ever played with action figures you know you have to get them balanced just right, or they would topple over. Our muscles keep us from toppling over, but that requires a lot of weight to be balanced on certain points on the anatomy. The head weighs about 15 pounds, and all that weight has to be balanced atop your neck. This requires lots of muscles working together to maintain on good days. When you slouch, or your shoulders roll forward, you are throwing off that precious balance that your body needs to keep you from falling over. You want to stay upright, so that new posture puts pressure and strain on those structures. When this strain happens your muscles get tight in predictable ways. This tension can cause headaches or any number of structural dysfunctions like thoracic outlet syndrome, upper back pain, or simple neck pain. It also works down into the lower back, throwing off the hips and causing low back pain and even the symptoms of sciatica.
In other words; Bad posture causes tissue dysfunction, which results in pain. It doesn't happen overnight or all of us as children would have paid closer attention when our mothers' made mention of it, (pain tends to catch everyone's attention, even kids.) but it does happen, and when it happens it takes weeks to fix. You can avoid the whole problem simply by being aware of your posture and making adjustments to correct it.
So next time you catch yourself slouching, thank your mom and sit up straight!