Now, if she had been the heroine of a moral storybook, she ought at this period of her life to have become quite saintly, renounced the world, and gone about doing good in a mortified bonnet, with tracts in her pocket. But, you see, Jo wasn't a heroine, she was only a struggling human girl like hundreds of others, and she just acted out her nature, being sad, cross, listless, or energetic, as the mood suggested. It's highly virtuous to say we'll be good, but we can't do it all at once, and it takes a long pull, a strong pull, and a pull all together before some of us even get our feet set in the right way. Jo had got so far, she was learning to do her duty, and to feel unhappy if she did not, but to do it cheerfully, ah, that was another thing! (42.14)
One of the things that makes Jo such an attractive heroine for readers of every generation is that she's far from perfect. First she has to learn what's right and wrong, then she has to gear herself up to do the right thing, but even then she's not necessarily happy about it.