It's an evolutionary trait that served us well for thousands of years, but has turned detrimental in modern times. We love eating fat and sugar because it was rare and difficult to acquire for most of human history. Now it's abundant and we have to literally fight against our biological instincts to not stuff our faces with junk food. Likewise there used to be no point in exercising for exercising's sake because just going about the business of staying alive was a very physical task. It was to our benefit to get things done while expending the least amount of energy possible because food was scarce and requires a lot of work to acquire. Now we must fight our instinct to conserve energy in order to go out and exercise so our bodies stay physically fit. All this technology and convenience has really fucked us over in some ways.