guitar strings ?? & piano

After being played for some time most guitar strings sound dull and when replaced with new ones they sound brighter and louder. My theory is that after a string is played for a while it gets stretched and has to be retuned. This work hardens the string and makes it a bit thinner. Guitarists say that it's dirt, sweat and oil form the skin that causes the problem. I'm not into piano but IIRC correctly they get tuned once twice a year and strings are only replaced if they break. One reason might be that they are struck with a soft mallet. I'm intesested in the sience behind this. tnx