"When women are speaking at great length, men tend to tune them out."
Apparently there is a scientific "reason" that I hesitate to share because it will only encourage these beloved, baffling creatures to continue swearing up and down that "you did NOT tell me that! I do NOT remember you telling me to do that!" But in the interests of science, The Haggis shall provide:
Female voices, a new study finds, are more complex than male voices, making men's brains work harder when listening to them. The shape and size of the female vocal chords and larynx lead women to speak with a greater range of sound frequencies than men. Women also naturally have more melody in their voices. Listening to female voices activates the auditory center of a man's brain, which must analyze the different sounds at the same time he's interpreting the words. Men process other male voices in a different portion of the brain.
Which I guess confirms that a man can focus on only one aspect at a time: either content or tone. Which explains why he can be turned on by a breathy sultry voice reading the phone book OR completely miss the sarcasm when you say "Of course I'm not mad at you." ;)