Most of the time, Cherrim have a hideous appearance. their frowning faces hidden perpetually from view beneath a cocoon of purple petals. Yet when the sun is bright, the petals turn pink and blossom, and they rival Blissey in happiness.
One can find a simple explanation for this fact in most species, for sunlight provides essential nutrients even in humans, and far more in plants. And Cherrim, pink in color, are less adept at photosynthesis than other grass pokemon. But this theory could only explain Cherrim's transformation in terms of hibernation, it does not explain the dramatic shifts in mood. And Cherrim with their petals draped over them are very much awake, as many a trainer can attest, but they are as gloomy and despondent as Yamask.
This nutrient-based explanation was the source of decades of fruitless research, but it is a red herring. Sunlight does not only provide nourishment, but also light, and long, dark nights are seldom appreciated in creatures which rely on sight. Cherrim's eyes in particular are quite tiny, and they do not respond much to artificial lighting designed for humans. They can claim no nose, let alone the sharp sense of smell that guides most cave-dwelling and nocturnal pokemon, and although they can understand trainers' commands, science has not yet located their ears. Their limbs are too short for touch, and although it is more developed in Cherrim than in most plants, taste is an awful way to navigate. The Cherrim are happy in bright sunlight for it is the only time they can experience the world around them; their despondence stems not from despair, but from sensory deprivation.