Funny part of being kicked in the nuts?

My cousin kicked me in the nuts really hard because she "felt like it and it's just funny" and I dropped like a sack of potatoes, and I hadn't annoyed her or anything she just genuinely couldn't stop laughing. She ended up feeling bad and apologising but kept joking about it

So I'm wondering what makes it so funny?

Funny part of being kicked in the nuts?
Post Opinion