A moment of complete and pure sadness is a blessing. I could defend this statement with the same argument as one might use to defend the darkness. Without darkness we would not value light. Without sadness we would not value happiness. I could write about these two opposing forces and the harmony they accidentally create. Instead I will write about how humans, when devastated, find their core.
For example, we are quick to share good news with just about anyone, but when the news are bad we tend to be more selective. Therefore, sadness makes us value those who truly matter. This is just one of many examples as to why sadness is the most human of all sensations. We tend to be sad because of expectations that we set too high. We are sad because of ourselves. No one can ever be blamed. We are sad because we are disappointed with someone or something, but it is our own fault for trusting in them. We created an illusion to accommodate ourselves and then regret it. No other living being does this (that we know of).
When we are sad we learn what we do not want to experience and we discover who can protect us from such things, or at least ease our burden. Humans, when sad, become interdependent, or at least acknowledge that they are.
This is why sadness is a blessing. It shows us what we really are, how we can be brought down or how we can be built up again. Sadness, in its essence is a gift, for it shows us just how human we truly are.