Why is it difficult establishing causation for allergies?

[Mpatton]

I know the immune system can become hypersensitive in the presence of allergens. But why is it difficult working out the cause of allergies? Is there a specific reason? Who is at risk and what are the challenges preventing effective intervention and control? I've done quite a bit of reading around these questions so I know some of the basics but I'm not quite sure which angle to take answering it....any ideas?