There are a few theories around, mostly around the changes in lifestyle in the last couple of hundred years compared to what we're evolutionarily accustomed to.

There are a few theories around, mostly around the changes in lifestyle in the last couple of hundred years compared to what we're evolutionarily accustomed to. Things like not walking/exercising nearly as much as we once did, a much more refined and sugary diet, much less exposure to pathogens (lazy immune system theory).