Have you met people who are going for holidays, and they're so nice they take a order list from others and they end up helping people buy stuff the whole holiday?

I know I have.
I have nothing against people like that. T
hey're too nice but if they're happy being too nice, by all means go ahead.

What I can't stand is when these people assume everyone is as nice as them.

They make unreasonable demands such as "Can you buy this and this back for me? I want to give to my colleagues. Of course I will pay you back! Just want to treat my colleagues to some food from Taiwan."

Excuse me? Why should I help you carry your stuff back so you can "por" your colleagues?
The whole concept of souvenirs buying is when you are physically there and you buy back for people to show appreciation. Who are you to tell me to buy things back for you so you can por your colleagues, especially when you're not the one who's physically traveling.
Oh and do you know, food is heavy? Very heavy? 

This is not logical. Not logical at all.

I wish these people will come to their senses and realize that their requests are actually quite unreasonable.