Why do I get more sad during the winter?

I'd like to start this post by saying that I love life. I love the unexpectedness of it, I love the awesome people we get to meet, and how these people can be so different but all have things in common at the same time. I love how we get to learn from our experiences, and those of others, and how we get to make mistakes to hopefully learn from them.

I am soooo passionate about life that I want to do the most I can in the short lifespan we get to live as human beings. I love life so much that when I die it will certainly be either because I'm too old, I died from an illness or someone killed me, but I will never take my own life. Even when I have days that I feel down, and when the world feels absurd, and when I question existence, even then I still love life. However, I have noticed that especially during the cold winter months I tend to be a little down, I tend to have less energy, and feel a little sad. Why is that?

Is it my imagination? Do I seek an excuse to feel this way by blaming the weather? I am not sure, but I do feel that cold weather does not help. In the summer I go out a lot, enjoy nature, enjoy the company of people a lot more, feel full of energy,  feel like dancing, and jumping, and twirling, and what not.

When it's cold out I don't want to get out of my bed, or walk around, and this lack of activity seems to have an effect on my mood. At times I can barely stand myself, and then I want to get isolated. But then this attitude makes me wonder whether the weather is the real problem, or whether there is something deeper that is wrong with me. Am I feeling like this due to frustrations that I have been storing in the back of my brain, until I am not occupied with other things, and suddenly all the frustrations come out due to the lack of external motivations? Am I feeling like this because I am in a new environment, because things have recently changed, because even though I know there are people that care about me, and that are there for me, I often feel like I don't belong anywhere, and that I will never fit in to one place in particular?

I stop myself from thinking too much like this, and after a few days of feeling down I tell myself that this has got to stop, and to think about all the good things going on in my life, and to remember how much I love life and how passionate I am about it. I also tell myself to stop being selfish, and that the world does not revolve around me. Yes, my feelings are important, but so are the feelings of the people around me, and I know that feeling like this makes people around me sad, so if I care about these people I will get up, I will do what I need to do, and I will do my best to make the life of those around me, especially those who care about me the best I could possibly make it for them. I will try to waste less time on Facebook, and try to finish my work for school, and spend real life time with those I love.