Seriously, where is your friend from? Is english a second language? Are they an engineer, or a high level surgeon? A Librarian? My mother-in-law?
Sigh, I can't imagine not understanding sarcasm, It would be like a gigantic social joke that was lost on you.
I would say something like: Sometimes saying the opposite of something, makes it funnier...because it is usually very obvious that it is the extreme opposite...but there are other forms of sarcasm.
You can offer, the other reason people use sarcasm is to show their contempt, or disrespect, or even their doubt for another. It comes into play heavily when two parties have a difference of opinion, or point of view. This is why we see it between comedians and politicians, or between siblings or close friends...and especially enemies.
sar·casm? ?/?s?rkæz?m/ Show Spelled[sahr-kaz-uhm]
1. harsh or bitter derision or irony.
2. a sharply ironical taunt; sneering or cutting remark: a review full of sarcasms.