Which means you have to change the strings or it sounds crappy, even more so than normal.
So I changed my strings today. This also means that for the next several days, depending on how much I play and keep re-tuning, the new strings will keep stretching out and going flat, until they reach a point of relative equilibrium. Steel-string acoustics don't seem to take as long as nylon stringers do.
Instrument strings tend to keep stretching over the course of their lives and when they stop, assuming they don't break, they start to sound dull and dead.
How you know they are dead for sure? Put your guitar away for a few months, then pick it up. If the strings are still in tune? They's dead, Jim.
There is a sweet spot, between going flat because they are new, and dying when they are old and corroded, during which the tone is as clean and bright as it will get. Different players have differing sweet spots, depending on a number of things, including how hard they hit the strings, the climate, the acidity of the sweat in their hands, what kind and gauge or tension, the strings are, how long they practice, all like that.
(There are players who are considered toxic–their sweat will corrode metal strings after a couple of times playing, so they have to either change the strings or use special cleaners to wipe things down after every session.)
There are guitarists who change strings as infrequently as possible, a year, two, longer. Others will put new strings on every month, sometimes every week, especially if they are doing concerts.
Me, I'm usually good for four or five months, given my daily time and the relatively quiet levels at which I play, but when it's time, it's time.
I broke a new string changing 'em today. That's rare, but it happens. Fortunately, I had extras.
There's an old story of a guy who goes to a swap meet and finds a really nice guitar for sale really cheap. He picks it up, but the tuners are frozen. Guy selling it says, "Yeah, it's my brother's. He bought it, then got tired tuning it every day, so when he got it sounding the way he wanted, he super-glued the tuners in place ..."
Players will think this is funny. Non-players won't get it ...