When asking someone to do something, there is a big difference between a favor and a challenge.
A favor requires the requested to invest time and effort on behalf of someone else. When it comes from a stranger, its success tabs into the altruistic side of the request recipient.
A challenge, on the other hand, tabs into the competitive side of human nature. You can argue that Jackie accepted my request for Olympic ring donuts partly because it was both a favor and challenge.
In my case, because I ran into two people who are training for competition and are probably very competitive in nature, they accepted without hesitation.
Learning: When requesting something, consider turning a favor into a challenge. For example, when you ask for donation to charity from strangers, say "most people won't donate to charity through a stranger, but we would appreciate it if you are different and can help."
However, make sure to make it known that first and foremost, what you are requesting is a favor, so it doesn't turn into anything manipulative, which is both unethical and could backfire.