The secret is the mirror neurons.

If you see someone doing something, like eating, crying, having a pain, you will have the same neurons light up in your brain. This is the same that happens in a horror movie, you get hurts even if you only see what happens .

For this reason, if you see someone doing something you want to learn, allows you to learn much easier and efficient (and that was the way you were learning when you were a baby).

Another example, is to see someone moaning will put you in a negative feeling (and the opposite when you are with someone extremely positive and happy).