They also grew out of a way of showing friendship. Money doesn't buy love or friendship. Don't push the issue - you might not be as close to someone as you think, and it's no fun to find that out. There are several points of etiquette on both sides of the equation. But if you can't draw, don't insist on trying.
Some artists are perfectly willing to do sketches for no consideration.
Be very nice to these people. They think that having other people see their work in your book will be reward enough. However, just because they do it for someone else doesn't mean that they owe you anything. It's perfectly natural for an artist to give preferential treatment to his friends. Themes can be fun, if they're flexible.