Sadness can be cleansing and enlightening.
Buddha's parents tried their best to keep sickness, death and unhappiness away from their son. Yet, eventually he found out what happened beyond the pristine and artificial palace walls. His reaction was extreme; he ran away from his throne, his wife and even his child to ponder on life's questions and challenges in the forest. Who knows, had his parents exposed him to life instead, maybe he would have followed in his father's footsteps.
I came across the book, "The upside of your dark side", by Todd Kashdan and Robert Biswas-Diener. Their conclusion was music in my ears: while they acknowledged the psychological benefits of positive thinking, they argued that by embracing, and constructively using the entire spectrum of our feelings - positive as well as negative - we can do even better. The "high" is more productive and powerful after the preceding melancholy; and the sadness and depression may give us much needed downtime and soul searching opportunities.
That's indeed the message of the Way as well. Everything that life brings our way is perfect for our soul development and should be accepted and opened up to; even if it doesn't look so appetizing on the first sight. We can be God's vessel that stands ready to integrate everything life brings our way. The wisdom of Zen is, "it is as it is"; while the advanced version of this insight may read as, "it is as it is, and as is, is good."
Rumi illustrated this philosophy in his Guest House poem nearly a millennium ago:
This human being is a guest house. Every morning a new arrival: a joy, a depression, a meanness, some momentary awareness comes as an unexpected visitor. Welcome and entertain them all! Even if they is a crowd of sorrows who violently sweep your house, emptying it of its furniture; still, treat each guest honorably. He may be cleaning you out, getting you ready for some new delight. The dark thought, the shame, the malice; meet them at the door laughing, and invite them in. Be grateful for whoever comes, because each has been sent as a Guide from Beyond,