Are some people not cut out for life? Maybe not. Survival of the fittest seems to imply that life is hard for all species, and not all conditions are conducive to the each organism that is living. Not all can adapt or be resilient.
It seems to me that all species seek first to survive and then to thrive; when they can no longer thrive, they revert back to survival, and sometimes they cannot even survive.
Life is a fucking battle for every species. We as humans are just more conscious of our experiences, whether positive, neutral, or negative. We don't just experience them, we are more highly aware of them, and that can feel like a curse. Philosophy is all about that awareness, and seeks to understand it and figure out what the fuck to do with it. Many philosophers have said that the point of philosophy is to learn how to die, and Camus said the point of philosophy is to decide whether life is worth living.