Theists often argue the following:
Our universe and its fundamental constants are designed by God to allow the perfect situations for humans.
Setting the presupposition of God's existence aside, this argument has many problems. It is impossible to reconcile an all-perfect and all-loving being with our universe.
Objection 1: There are no other universes to compare to
Why is this universe a perfect one? Other universes may be more suitable for life or may have been designed to be better suitable for life. We only have one planet in which life does exist to actually observe, while other planets that may have life may be better suitable.
Objection 2: The eventual heat death of the earth
The sun is eventually going to fizzle and all forms of life on earth will be extinct. Earth will be uninhabitable. Some design.
Objection 3: Evolution is a fact, not design. We don't need a designer to explain the evolution of lifeforms.
Evolution sufficiently explains how life progressed and is a more than plausible naturalistic explanation of life. We can't just automatically say "God started the universe" because we don't know the facts. We don't need an intelligent designer or creator of the universe to explain it.
Objection 4: Natural disasters, viruses, harmful mutations, and spontaneous abortions are contrary to a perfect universe.
Earthquakes, tsunamis, hurricanes, and other natural disasters are natural occurrences that destroy life on the earth. A perfect universe created for humans and other life should not include disasters like this. Diseases, genetic abnormalities, and birth defects ravage life. Spontaneous abortions show that the human body is imperfect and that births are not sufficiently designed to occur without issue.
Objection 5: 99.9% of everything that ever existed is now extinct.
Throughout history, countless lifeforms have been utterly destroyed by natural means...and humans weren't even the starting lifeform. If the universe was designed with humans in mind, millions of years have passed without them while other lifeforms have passed. Some design.
Objection 6: Humans lived in horrible conditions from their onset.
Early humans have died from simple diseases, tooth decay, terror about the universe, natural disasters, epidemics, plagues, and constant warfare.
Objection 7: Lifeforms aren't suited for the earth, but rather life adapts to the environment.
Evolution by natural selection shows us that lifeforms best suited to the environment survive, not that the universe was made to have lifeforms survive.