Found on sabren.com
They say that if you place a frog in a boiling pot of water, it’ll jump right out. But if you put the frog in lukewarm water, and only bring it to a boil slowly, the frog will not notice the slow increase in temperature, and will happily let itself be cooked. Why anyone would like to eat a frog is beyond me. However, the metaphor fits.
How happy people become miserable and don’t even realise it, one little thing after another leads to a steaming pot of boiled frog.