I think we all feel what is right and wrong in any situation. Doing what's right makes otherwise complex decisions very simple.

If you do what's right instead of considering your benefit or lack thereof, you just do what's right, even if you would benefit more from another action. When you feel what's right and do it, life becomes greatly simplified. Why'd you give the money back? Because it was the right thing to do.

If you lost your money, wouldn't you feel amazing if someone gave it back to you? Giving someone that feeling is worth the money. Keeping the money would make me feel terrible.