"Art is the emotional expression of human personality." -Eugène Véron, L'Esthetique (1882)
Art is truly amazing because of it's ability to express what words cannot.
Our connection to art is heavily based upon stimulus, which is commonly shown through emotion. Depending on the subject of the artwork, our reaction to it will differ. As it would seem, sad art evokes sad feelings in the same way that happy art evokes happy feelings.
Some days, we feel sad, and hearing a certain song can help us to feel better. This is because we relate to it, much like art. We see pieces of ourselves within it. Then, there are other occasions, where we use art as a medium for reassurance, because it's a tangible piece of emotion.
It is in these artworks, that we may see parts of ourselves, in the patterns and the colors, perhaps it's all the little things, and how it relates to us just clicks.
A common image in older artworks like Frida Kahlo's work, is flaws. Some of the ideas conveyed in her work were based around sadness and trauma, and she wasn't afraid to convey those ideas into her work. People who saw that art, may have related to the idea that no one is truly perfect.
If we were all "exactly" perfect, none of us would be different from each other. We'd all be identical, instead of embracing who we are and how we differ from one another.
We absorb and relate to what is in these creations, and we are shown what pure emotion can result in. This tends to both uplift and inspire us to use our own emotions to create and inspire the next generation while we're still impressing ourselves.
Don't forget how art has impacted the world and how it inspires us to create.