The cries of an infant are impossible to ignore. This is not a sentimental statement – I fully believe that we are hardwired to react to the sound of infants crying. That particular waveform triggers a cascade of neurological signals culminating in the area of the brain known as "do something about it now you bleedin' tosser". All in all, this trait is an advantage to mammals everywhere... except maybe with humans.
Sure, most of the time a crying baby lets you know that you've missed or forgotten something and, after remedying said problem, the crying stops. But then there are the days when you really can't do anything for the baby – like, say, when she has a cold. Then you get to listen to the crying during her every waking hour and the wiring in your head screams... "do something about it, do something about it, why the #%@*& aren't you doing something about it!" Meanwhile, the rational... well, conscious... part of your brain is trying to calmly explain "I did everything I can for her, it's okay, stop going into overload." Which is, of course, right up there with trying to explain to the rain that now really isn't a good time and could it please come back tomorrow.
Makes you realize that three isn't such a bad age after all.