"Do you want something to eat?" - "NO"
"Do you want nothintg to eat?" - "NO"
"What do you want?" - "I don't know" (off she run crying)
That was a conversation between me and my 3 years old daughter.
As she run away, totally overstrained by the situation, her dad and her world over all, I had pity on her.
And one thought further, I recognised the true answer she gave me:
it's not "YES", it's not "NO" (regarding the 1st question), it's "NULL". Or translated into what I think are the thoughts of a 3 years old girl: "I don't know what to do now, so I'm against anything and run away!"
From now on, if any application designer argues with me, why any column can not be set NOT NULL, I will remember this picture of a overstrained, 3 years old girl and just feel pity.