We have said that humans can live under the most varied environmental conditions but this doesn’t mean that they should. From the existential point of view, everyone has their own unique potentialities, their own Self that they are capable of becoming, and from this point of view the goal should be to make environmental conditions as conducive to that trajectory of growth as possible. When conditions don’t meet needs and potentialities the result is ill-being, which manifests in the wide array of mental illnesses.
The best way to understand what we’re talking about is by making the comparison of the growth of various forms of vegetation in a garden. Each seed can only become the plant that it potentially is, although all these seeds look pretty similar before they’ve germinated. But they have different potentialities, different fruits that they will one day bear. But these potentialities will only manifest if they are given the proper amounts of nutrients, light, water, etc. As every gardener knows gardening is not a one size fits all affair. Different types of vegetation have different needs.
So while from the behaviorist’s point of view humans will simply adjust themselves to changed levels and intensities of reinforcements, existentialists realize that while this adjustment is possible it leads to bad side-effects, to ill-being, just like how a plant that is not properly taken care of might still grow but probably won’t produce the same yield and it will look sickly, whereas the plant that gets what it needs will flourish and become that which it was meant to be.