A story which is too serious tends to be too heavy. That which is too light tends toward frivolity. An enjoyable work will juxtapose the two, carefully weighing one against the other until all is in equilibrium.
Characters must have moments of joy, in order to appreciate the loss when they are suffering. If all is misery, there is nothing to fight for. Such an existence is subsistence and nothing more.
Readers hope the best for the characters they love. They will watch that person descend into darkness, but wish fervently they be not consumed. As events conspire to drive a character to madness, the literary spectator cheers them on to overcome and retain their mental faculties.
Moments of levity inevitably sharpen the eventual times of despair. If you like that person, you will laugh with them and weep for them. And the more contact you have, the deeper your enjoyment will be.
Even the individuals that are not likable can be understood. And thus, it behooves the author not to drown their reader with too much emotional turmoil. You must keep the pressure up, without crushing the enthusiasm. For it is through excitement and enticement that a reader can be happily drawn into a literary trap.