Spring is the season for suicide

Contrary to what I (and probably most other people) have always assumed, it seems that statistically most suicides actually occur in the spring.
While this may be counter-intuitive to most people (spring being the season of new life, renewal, hopefulness, improved weather, etc), most longitudinal studies in Europe and North America seem to confirm it. In Canada, suicides tend to peak in May, with fully 21% more suicides than in the month with the lowest suicide stats, which turns out to be (again counter-intuitively) December. The spring suicide phenomenon even applies in the southern hemisphere, according to Australian data.
But no-one really seems to know why. Various hypotheses have been put forward to explain this phenomenon, including: that it is a delayed reaction after the gloomy winter months; or that the brighter, symbolically more positive days of spring throw a person's depression into even more stark relief; or even that it is due to sleep disturbances caused by the spring clock change. But none seems entirely convincing or definitive.
The latest hypothesis, still under investigation, is that suicide rates are linked to pollen counts from popular hay fever allergens like flower, grass and tree pollen, which also tend to peak in spring and early summer. Hungary is a particularly cogent example: it has one of the highest suicide rates in the world, as well as one of the world's highest ragweed pollen counts, and the suicide incidence in Hungary peaks notably at the same two times of year as peak pollen production.
This unlikely-sounding connection is not actually as far-fetched as it might seem. Allergies cause, in addition to the usual external nasal symptoms, a spike in the production of a kind of protein called cytokines, which are able to pass the blood-brain barrier, and which have been separately linked to depression and suicidal tendencies through a variety of possible mechanisms (including the suppression of serotonin production, and the activation of microglia immune cells).
At the moment, the link, which has been indicated by several, but by no means all, studies on the subject, can not yet be considered causal, and many more studies need to be done. But I was intrigued by the thought that something as apparently trivial as hay fever might be linked to something as dramatic as suicide.