Fun Facts: Why do some people have seasonal allergies?

Seasonal Allergies

Asthma and allergy rates are on the rise across Canada, according to a chorus of environmental experts and advocacy groups. Mounting evidence shows that the hay-fever season now lasts up to a month longer than in the past, and parts of Canada that were once too cold for ragweed and other allergy-inducing plants are now seeing activity for the first time. (article)

The economic impact of seasonal allergies has been pegged at 5.3 Billion in the US and upwards of 7 Billion in Australia (no numbers are available for Canada at this time). So why do they happen?