The Cells In Your Body React To Everything That Your Mind Says. Negativity brings down your immune system.
This might be true, but the 'you ought to feel and think like this or else' commandment of Western positivity culture can cause others to feel guilty for their inability to get past their current thought process, thus causing further negativity and, as this post points out, further health issues. It is healthier to not deny a feeling whether it is positive or negative. Trying to squeeze positivity into a mind rightfully afflicted with a natural emotion is also a dangerously unhealthy habit that one needs to overcome.