Lots of people are depressed around this time period to the point of wanting to kill themselves. Also, people who are not in the best of health just feel like giving up for some reason around Christmas. Maybe it's the weather. Maybe it's them looking around and being depressed that they have no loved ones in their lives. Peace
I think those factors are involved, along with Seasonal Affective Disorder (SAD). This very real depression is caused by a lack of sunlight, which peaks at the Winter Solstice which in the Northern Hemisphere occurs around the 21st of December. SAD is treated by placing a bright wide-spectrum light source near the patient for several hours a day. Obviously, they didn't have a light souce like that in ancient times. It is for that reason (I believe) that Christmas and many primitive festivals (some featuring 'light' as a focus) are held on or near the Winter Solstice. The ancient Christian churches don't claim that Dec 25th is the actual birth date of Jesus. The date was officially adoped by yhe churches as the celebration of the birth of Jesus about 350 AD.
http://en.wikipedia.org/wiki/Winter_solstice.