There’s a fine balancing act in the brain and circulatory system that regulates our moods, boosting the feel-good factor or sometimes causing Seasonal Affective Disorder (SAD) in winter. The hormone melatonin, produced by the pineal gland in the brain, regulates our sleep/wake cycle, but excessive levels can cause depression. The action of light on our face suppresses melatonin synthesis, therefore boosting your overall mood. Evolutionary principles could apply too. There are plenty of nocturnal primates and presumably they aren’t always depressed. It seems likely that we have evolved to prefer light to darkness because humans are active during the day.