To Resign...or Continue the Fight?

Let's say you're losing.  OK...let's say you're losing BADLY.

What is the tipping point in your mind that...

decides to either fight or flight...resign or fight on??

I've played in games where I may gain a microscopic advantage.  And MUCH to my surprise...my opponent actually resigns!!!  VERY STRANGE.

Then I've had the other extreme, with the player waaay down with really no chance...but still the game goes on and on. 

What do YOU think?