Is it the diet Coke or the lifestyle of the person who consumes it in significant quantities?

I've noticed that during healthier times of my life when I'm experience less stress, I have no interest in junk food or soda. When I get stressed and I'm not sleeping as well, I get more cravings and find myself making unhealthier choices for the little satisfying bump a dose of sweet/salty/fatty snack and a caffeinated soda provide. Obviously living a stressful life is unhealthy as a baseline.