Q: I heard that Kai thought it would be funny and lift Jan's spirit if he pulled a prank on Jan, but it ended up really pissing her off instead. And I was wondering Why did Kai pull a prank on Jan?

OPT:
--think of pulling a prank on jan.
--piss Jan off.
--make Jan laugh so she wouldn't be sad.
The answer is:
make Jan laugh so she wouldn't be sad