Please explain how prawns and mint complement each other.

Depends how you're using them. I've made several Vietnamese recipes - e.g., rice noodle salads with prawns/shrimp - where it's not just about the prawns and mint. Rather, it's the balance of sweet, salty (fish sauce,) tart (lime,) spicy (chilis) and the herbal cooling effect of mint and/or cilantro. There's a magic that happens when they all get together.

I don't think they do, in general. However if the shrimp are in a spicy Asian preparation, I think the mint acts to cool down the heat.

Sounds like a need for the Flavor Bible. Check it out at the library or bookstore.

I've only ever had the two in Vietnamese summer rolls or Vietnamese prawn salad (goi tom). The mint, in addition cooling, brings out the sweetness of the prawn meat. That's the best way I can describe it.