Why did the other developer who passed you the bug not make an estimate then?

I understand the urge to quantify something that is impossible to quantify beforehand. There is nothing wrong with making a guess, but people who don't understand my argument usually also don't understand the meaning of "guess". A guess is something based on my current understanding, and as that may change substantially, my guess may also change substantially.

I can make a guess right now on any bug I will ever encounter, based on my past experience: It will not take me more than a day to fix it. Happy?