Pretty simple: where do you draw the line between when you are doing a friendly favor for someone (possibly doing free valuable work), and when you should be getting paid for your help (possibly being greedy or unfriendly)? Why?
Note that you don't always need to be paid with money. You can demand a good
liquor, a dinner, or a cake as your remuneration, depending on how big and
difficult the work is. And while it may be ridiculous to offer or demand
a mere twenty or fifty bucks, it's completely fine if it is a pack of good
beer.
If the other person will or could make financial gains from your help, then you should be getting paid. For example, if you build a website for someone to sell something, you should be getting paid.
