W.C. Fields was famously quoted as saying “Never work with animals or children”, and that wasn't because he hated kids or critters- it's because animals and kids are too hard to control.
They don't understand why it's so important for them to hold still while the photographer takes their picture, and they become even more rambunctious when mommy or daddy tries to take the pics.
All those amazing photos posted to Pinterest make people think it will be a snap to get the same results, but after a dozen or more shots they soon realize it's time to settle for "good enough".
See more about baby and kids at NeatoBambino