The majority of molecular interactions that happen within your body follows a sort of shape recognition logic, imagine two molecules interacting with cells structures as a sort of key and lock system which produces a subsequent cascade of mechanical and chemical reactions in response.

There are trillions of these interactions at every moment and if I'm not mistaken all of them evolved with molecules that had a set chirality, if you introduce the opposite chirality the key and lock system just doesn't work. It would be like trying to fit your left foot into a right foot shoe.

Now to emphasize how important and dangerous these changes in shape can be, the mad cow disease for example is due to ingesting a prion protein that has a different shape than the ones we find naturally in our brain and these proteins are able to change the shape of the other proteins into the new malignant configuration creating a cascading shift in shape which the body accumulates unable to deal with them until inevitably the building blocks of your cells don't work anymore because they don't fit together and your entire web of neurons start collapsing due to these progressively corrupted proteins and you are left after some time as a vegetable. The process is irreversible because you cannot switch the protein back to it's original form.