Any novel that doesn't plumb something of the depths of human emotions isn't worth the paper - or pixels - it's written on. Perfectly okay to do it in a subtle way, or to explore something light like laughter or wistfulness, but without some sort of meat, there's just nothing to sink your teeth into.
At the same time, plunging straight from heavy emotional moment to heavy emotional moment can quickly tire and bore readers. If ABSOLUTELY EVERYTHING IN THE STORY IS OF THE UTMOST IMPORTANCE, there's no break, no mental "period" at the end of a sentence of feeling.
The best stories follow a roller-coaster course from levity to darkness. Even a brief moment of humor can put the serious action before and after it in perspective - or starker relief.