It was the great philosopher who wrote "Why I am not a Christian". Reading his book as a teenager helped me to work out why I was a Christian, and what kind of Christian I was.
Since those early days being Christian has become even more problematic to me. Being Christian in an intellectually honest way I have never found easy. But I can with more conviction say why I am not a humanist:
Because human reality is not the whole of reality as we know it, and most of reality is unknown to us and perhaps unknowable.
Human beings are not obviously to be preferred to other animals or creatures. They're just more closely related. I believe my 185 million greats grandfather was a fish.