SAD Or Just Glum

It's the time of year when SAD kicks in for many people. Seasonal Affective Disorder? Why do we have to have a clinical definition for every mood we feel?! According to Wikipedia:

Seasonal affective disorder (SAD), also known as winter depression or winter blues, is a mood disorder in which people who have normal mental health throughout most of the year experience depressive symptoms in the winter or, less frequently, in the summer,[1] repeatedly, year after year. The US National Library of Medicine notes that "some people experience a serious mood change when the seasons change. They may sleep too much, have little energy, and crave sweets and starchy foods. They may also feel depressed. Though symptoms can be severe, they usually clear up."[2] The condition in the summer is often referred to as Reverse Seasonal Affective Disorder, and can also include heightened anxiety.

Why do we need an excuse for everything? I challenge calling this feeling a disorder. I think it's perfectly natural. Why not just say we're human and in the winter when the days are dark and gray we get glum? But when the days are sunny I feel better, or as in Wikipedia's definition, though symptoms can be severe, they usually clear up. Ha ha ha! Also works: a cup of hot tea and some chocolate!