I think facial expressions, body language, and tone of voice play an important as well. When my oldest was a baby my grandmother (twisted woman that she was) made him cry (ONCE) by saying postive things in a sad tone of voice.

Anyone with a toddler knows that if they fall the tone of your voice when you respond will have a direct response to how the child reacts. If you panic and say "are you ok" in a way that makes them think they should be hurt they're going to cry. If you say something like "boom, we all fall down (as in the game London Bridge)" they actually think for a second and laugh instead of cry. Or you tell a child they have to go to bed in 15 minutes you get a fight, but if you tell hey, I'll let you stay up for 15 more minutes you don't.

I don't think this is limited to children either. Kids crying, screaming, or thowing a fit go right to my spine. However kids screaming with joy, laughing, etc. no matter the decibel don't bother me a bit.

It's a challenge to outsmart their curious nature at this age.