... because in its entirety the film is very satisfying.
Mr. Polly (John Mills) is hard to root for. He is a lazy employee in a shop who gets fired for reading novels when he should be working. He spends months looking - not very hard - for another job when he gets news his father died. His father left him a small inheritance, and so he decides to open a shop himself, although he hated working in one. Then he decides he needs a wife. He falls for a schoolgirl ten years younger than himself, but it turns out she is just teasing him for the amusement of her friends, so he dispassionately marries a spinster cousin.
15 years later we are shown where Polly is - on the verge of bankruptcy, with his wife constantly nagging him about things that seem to be very true about himself. The shop is a failure, in part because it is chaotic - it does not seem to have a theme at all. So the unhappy Polly decides to commit suicide by burning his shop down with him in it while his wife is at church. Polly succeeds at burning the shop but not himself, and decides to hit the open road. And it is at this point he becomes a completely different kind of person than he has been for the first half of the film. In other words, he does become somewhat admirable - focused, not lazy. The odd thing is that Polly has no real basis for any of the actions he takes at any stage of his life. He certainly isn't doing anything because he wants to do so, and in some cases his actions make him unhappy, and in latter cases they lead to happiness.
The entire thing is very ironic as the first and second parts merge into a very ironic and short third act. It's definitely original, and John Mills always improves any film.