The legend says that if you drop a frog into hot water it will jump out at once, but if you place it in lukewarm water that heats up slowly it will stay until it is too late. Biologically the story does not hold, yet as a metaphor it captures a human truth well: we adapt to gradual decline so smoothly that we fail to notice when we crossed our limit.
Think of a job where the hours creep up a little each month, respect quietly erodes and stress becomes a daily companion. No single event is big enough to make us say "enough". The same happens in relationships that cool slowly, or in habits that wear us down step by step.
Psychologically, this happens because our brain compares today with yesterday, not with who we were years ago. Gradualness switches off the alarm. So many people realise how heavy a situation was only after they leave it and look back.
The antidote is to pause now and then and ask: "Would I accept this situation if it were offered to me suddenly, exactly as it is today?" If the answer is no, perhaps the water has already warmed. For the workplace version of this effect, see our piece on the toxic work environment.