Answer:
The concept of insanity can be understood by looking at the most common example available to all of us: falling in love.
The subject will be irrational to the point of self-sabotage, and if the relationship fails, despite the emotional suffering endured the first time around, will willingly put themselves through it all again if they again "fall in love".
Repeating an action indefinitely in the hope that the result will change despite all evidence pointing to the contrary = insanity.