Why does the male puffer fish do this? Because he's programmed to.
There are many things that the theory of evolution cannot explain. See the YouTube video "Carnivorous UNDERCOVER caterpillar 🐛😎 | Wild Isles - BBC".
The Return of the God Hypothesis
The Problem of Evil and the Skeptic's Dilemma
Can You Believe in Science and God at the Same Time?
The Problem of Evil and The Logic of Hope