Have you ever done a poor job on something? if you have, have you felt badly about it? It seems like a lot of times we feel badly because we didn't try very hard.
And conversely, we usually feel pretty good when we do our best. Unless we get a sense that others didn't feel like it was a good job.
Which brings us to the possibility that the best work we can do is work that we have an ability for and that we try out best at.