Why is fitness always relative?

Adaptation to the environment helps animals survive. For example, the color of many species is associated with their habitat (for example, tigers live in shady jungles and therefore have a striped skin). However, the conditions for the existence of living organisms are constantly changing under the influence of natural processes or human activities. In connection with this, animals have to re-adapt to a new state of nature. Species that cannot do this die through constant natural selection.