All his life he tried to be a good person. Many times, however, he failed. For after all, he was only human. He wasn't a dog.

The line hinges on a comic reversal: we expect “only human” to excuse moral failure, but Schulz adds “He wasn’t a dog,” implying that a dog’s loyalty, steadiness, and uncomplicated goodness can seem more reliable than human virtue. It gently satirizes the lofty ideal of being “a good person” by acknowledging how often people fall short through weakness, ego, or inconsistency. At the same time, it’s affectionate rather than cynical—suggesting that moral imperfection is part of the human condition, while holding up the dog as an emblem of simple, instinctive decency. The humor sharpens the pathos: striving matters, but perfection is unrealistic.