Rarely, a browser update will clear cookies. More likely, someone being helpful will suggest that you "clear your browser cache" in order to fix an issue with a site, and if you use the default options it will clear cookies too. The connection might not be obvious (the broken site starts working, you go on your way) but then you'll have to login again at other sites. If you're like me, half the time when I do have to login, I forget to check the "remember me" checkbox...
For those that are choosing to not store cookies in order to be tracked less, there's no real point in that anymore as there are other, more reliable, ways used now as the primary means of tracking you. Google "browser fingerprinting" if you want to know more.