I rarely ask for favors. I don't like to impose on people. And really, I'm too particular to have others do things for me.
A "favor".... some people cringe when they hear that word. A bother. A pain in the ass. An inconvince.
To me it's an honor. A gift. A chance.
Let's think about it, you have obligations that you cannot meet and you are sending a proxy. By asking a friend to do something for you, you are saying, please BE ME for a little while. What an honor.
Think about it the next time to get pissy with a friend in need. Whether you think it in your head or say it in a grumble after the fact.