How can sunshine lift your mood?
Well, whenever there is sunshine, I feel a lot more optimistic, have more clarity of mind, feel more peaceful and full of energy. 
Why is that happening? If you look at people's mood in winter, they're always more gloomy, introvert and don't socialise as much as in summer. In summer, however, people go out, enjoy the nature, get more fresh air and natural light, interact more with other people, eat more fruits and veggies and, since they are in season, they are tastier. All these are no brainers for boosting our wellbeing and therefore our positivity overall. 
No wonder that sun is considered the source of life not only in contemporary science but also in many ancient cultures. 
I wish we had more sunny days and time to enjoy them!