Myusernameis - not dissing your point about pesticides, air pollution etc, but i think the people who focus only on those are missing a big, biologically relevant part of the story: your intestinal microbial ecology.
As well as air pollution and pesticides, the other thing that's really common now is people eating a lot of highly processed food, often without much fibre. This is something that has been an obvious change in the last 1-2 generations, whereas really high levels of air pollution and pesticides have been around for longer, more like 3 generations for pesticides and about 6 for air pollution in some cities. Obviously different biological mechanisms take different lengths of time, but processed food is on the right timescale to correlate with the huge rise in allergy.
There's been some cool research recently (only recently been possible, due to changes in how people sequence genes) showing that gut flora in people who eat unprocessed, high-fibre diets are very, very different from gut flora in people who eat the white-bread white-rice white-pasta packet-sauce processed-meat shop-cake western diet.
Now whether this is down to high-fibre or other aspects of the diet, we know that gut flora have a huge knock-on effect on general health. So if you change what food you eat, you may well mess with your gut flora, and what they're doing and how you're reacting to it.
Some recent research has shown that you can eliminate peanut allergy in mice by changing their gut flora.
There's also recent research showing that you can significantly reduce asthma, in mice, by feeding them high-fibre diets.
So there may well be a strong relationship between gut flora and allergy...