Unmitigated joy, unmitigated gloom... and our inner art critic

Art can be pleasant or disturbing. There are images that are pleasing to the eye and sounds that are pleasing to the ear; and then there are sounds and images and stories that are dark, dissonant, startling, unsettling, bleak.

Sometimes, pleasant and disturbing elements are both present within the same work of art, and the contrast or tension between the two is what gives it its emotional power.

For a work of art to be meaningful and emotionally resonant, does it have to have that contrast, that tension? Or is it possible for a great work of art to be entirely pleasant, without any negative elements? (or entirely disturbing, without any positive elements?)