Strings wear out. They lose their tone, become hard to keep in tune, lose their sustain, and break.
How quickly this occurs varies, based on how you care for your guitar, the environment your instrument is exposed to, the quality of your strings, and how you use them. I cover more aspects or ensuring the longevity of your guitar and your strings on the Guitar Maintenance page. However, even with the best quality strings, with the best preventative maintenance, in the best environment, your strings will eventually need to be replaced.
Thankfully, new strings don’t cost much and changing guitar strings is not that hard to do, once you understand how.
Bob Taylor of Taylor Guitars has has written an excellent explanation on how to change acoustic steel strings. You can get the PDF file on the “New Improved” Taylor method of changing steel string by clicking here.