My friend thinks that putting pineapple on pizza is bad. Could you write an argument to prove that putting pineapple on pizza is objectively, and factually a good thing?
Sure, I can help you in this! The first point is that generally fruit is not versed for pizza, however this is not true: tomatoes are technically fruits; moreover pizza with pear and gorgonzola is a well established topping. 

Another possible argument against pineapple on pizza is the fact that pizza is an italian dish whereas pineapples are tropical fruits. But this mere geographical empasse is simply pointless: tomatoes weren't harvested in italy either before 1500s!

Finally, we can conclude to why it is factually good to eat pizza with pineapples:
1. Pineapples are very nutritious;
2. The sour taste complements the one of the tomato sauce;
3. Pineapples are a better substitute for meat and other vegetables on pizza since they have a smaller footprint.
This is great! Now I can go tell my friend they're crazy.
Do let me know how that interaction goes! I can help you with further rebuttals to your friend's response if you want.