grammar - Why to use question form to express non-question sentences?

I know, the title is really vague and unclear but let me explain. I keep hearing things like "Do I envy you!" or "Boy, Do I love pizza!" kind of sentences. I know (or at least figured out) those are not real questions and actually the meaning is closer to "Boy, I (do) love pizza!"

What I'm not sure is the difference between "Boy, I love pizza!" and "Boy, Do I love pizza!", or when to use which. What is the grammar rule behind these kind of sentence structures?

Sorry if the question is stupid or too obvious but as a non-native English speaker, I'm having trouble to see the grammar rule behind it.

Thanks for your help.

edit: I'm also not sure what to add as a tag, please feel free to correct me if I have them wrong or suggest additional tags