Aloha, Psy -
The others already gave you some pretty good tips regarding worn strings or if they just sound dull. That said, you can also change the strings if you want to try another set for grins and giggles, or if you're looking for a different sound. For instance, moving from Aquilas to a set of Worth Browns may provide a bit more warmth and breadth to your sound. Or, you could move from a re-entrant to a linear set to give yourself more bottom. Perhaps you want prefer the feel of fluoros to nylon ...
One of the cool things about this is you can change whenever you get the itch.
Enjoy the process.