Why is seasonal affective disorder more severe in the north?

SAD: Sad is more prevalent in regions with colder darker wonters allowing for less interaction with other humans and the environment.

More prevalent: Perhaps you mean that seasonal affective disorder is more prevalent in the northern hemispheres -- not really more severe. An individual case might be similar in severity, no matter where the person suffering it lives. But the condition is more frequently seen in northern latitudes -- where in fall-winter, daytime light exposure is less than it is in southern latitudes.