The problem is light. The sun comes up somewhere after 8:00am and is gone by just after four. The journey to and from work is mostly in the dark. If you work in an environment where you don’t go or see outside, December, January and February can be somewhat bleak. Couple that with the weather which is dull in December, downright dreich in January and the aftermath of snow in February, we can see why s.a.d. is a thing. The dreich bit you can apply to any month. July, meant to be the opposite, can still be rainy and grey, but it does get light at around 4:00am and dark around 11:00pm or later. The further north the less light there is. The opposite of now.
I can see what HDT is saying. The odd still winter month, with no wind (rare), white snow on the ground, sun at its highest point, people sledging, is a lot like a cold summer. It is joyous. But the hours we have to play with are very short. The sun still warms, the crow still caws; if he has a voice, I have ears. And the wind still whirls outside as the rain makes it way east. It’s dark, oh so dark and about as far away from summer as you can imagine.