What makes a camera lens gritty/dirty?

My camera used to be super sharp and clear - but now it's all gritty... could it be from using an old lens cleaner? Or from air?? I had this same problem with my old camera after I took it down to TX (VERY HOT if u didn't know that lol) and that's why I got rid of it and got this one. Now the same thing has happened. Is this just some sort of scheme to get you to buy new cameras or something?!?!