Its kinda a hard thing. I'm a parent, and your first desire is to keep your kid safe and happy. Who really wants their children to be unhappy or wanting? That being said, it is vital that children learn to deal with issues related to loss, not getting what they want, etc etc. You are correct - its part of being human.

But its hard sometimes to not step in and try to always "make things right".

It isn't that I want my child to be unhappy. I want them to learn to be a well functioning adult. If they haven't learned to deal with adversity and you manage to protect then all the way through school all of the "good stuff" you have provided will be blown away by the first workplace bully that comes along. I want my children to be strong, intelligent and powerful when it comes to dealing with other people. They need to know how to deal with adversity and disappointment long before they have to deal with it alone in the real world.

I don't think any parent would intentionally wish ill intentions upon their kid. That, you don't have to worry about. What I do see, is that most parents' impression of "good child rearing" is to shield them from the world. You will find more and more examples of this than examples of parents watching their kids "experimenting with fire".