I will tell you a story of a little young girl. This is a metaphor to illustrate how serotonin works and influences mood and I enjoyed it, so here it is:
There was a little girl enjoying the long days and hot nights of summer. All day she was sitting under an umbrella and from time to time she used to look up to the sky and the sun. Taking a deep breath she was saying: "Ah long days and hot nights of summer. What a joy for me." And without anything else, after this she always felt an energy surge inside here that she would feel all day along the summer.
As days go by she started to think: "Hmm this serotonin hormone, is it the summer hormone? I feel so shinny during these days. Could it be it?" So every time she asked this, she just remembered the great feeling of summer days and nights of energy and good mood that she had.
Days came and days past without notice that white flakes were starting to fall under her eyes. Winter made it and here she was right in front of her. She started to wonder what is happening now with serotonin: "Will my serotonin level will disappear?" And days come, other were past and she noticed the the great feeling of energy was there, but much subtle than in summer days and nights. The feeling had transformed into something else in the same time. So she said to her self: "Ahh winter days and nights have calm my serotonin levels. Still it is a fussy feeling that I can enjoy, as long as my mood is good."
This way she discovered the what matters now is not what is serotonin, instead how it works during the seasons and it just transforms it's nature and effect.
So, days came and days gone with our little girl enjoying days and nights of summer and winter with all the fuzzy feeling that each season was giving her and embrace it with joy and stable mood.
Just as our little girl, we can understand easy that this hormone is here to balance and transform our feeling in good mood, suitable for each time of the year around.