It's called SAD Seasonal Affective Disorder. Meaning the season may have an affect on your mood, or you may be upset because many people feel as if they can't do anything because it raining outside. There's lots to do. Enjoy it XD
Many people love rain and are emotionally affected by rain for sentimental reasons. I can feel my heart calm and reminiscent of past memories seeing rain falls. Many songs related to rain are also sentimental, such as 'Rain' by Uriah heep 'November rain' by Gun's & Roses, 'Blue eyes crying in the rain' by Olivia newton John, 'Crying in the rain' by A Ha, 'Rainy days and Mondays' by Carpenters and so on.