Why have animals (especially humans) evolved to feel emotion?

Sometimes this is more obvious, like when a gorilla gets angry and defends his little gorilla babbys. But what about emotions such as sadness? Speaking of sadness, why have we evolved to cry? Isn't crying kind of bad for us since it messes up our vision? I would have asked this in biology, but I would get no freaking responses... And this isn't one of my stupid questions, I seriously want an explanation. So try your best to answer this pwease.