If pride is the first trait we notice about Margaret Hale, then impatience has to be what we notice about Mr. John Thornton. For a writer like Elizabeth Gaskell, Thornton is a classic example of "new money," a person who was born poor but has become rich. This is, in family history-minded 19th Century England, considered tacky. No, it's not fair.
Thornton has a vivid memory of how horrible it is to live without money, so it makes sense that he spends almost every waking hour thinking about how he can get some more. As the narrator tells us, "Mr. Thornton was in habits of authority himself, but she seemed to assume some kind of rule over him at once. He had been getting impatient at the loss of his time on a market-day" (1.7.13).
Being a self-made man, John Thornton has little time for people who complain about being poor. The way he figures it, everyone should be able to pull themselves up by their bootstraps just like he has. As he says to Margaret Hale, "I believe that this suffering […] is but the natural punishment of dishonestly-enjoyed pleasure, at some former period of their lives" (1.10.25). It's totally the old ant and grasshopper story. For Thornton, people who spend all their time playing instead of working deserved to starve and freeze come wintertime.
Mr. Thornton is a rugged individualist. He thinks everyone is responsible for their own destiny, and he doesn't care much for people who try to step in and force him to treat his workers a certain way. When questioned on his business practices by Margaret Hale, he replies that he is like most Milton men, in that "We wish people would allow us to right ourselves, instead of continually meddling, with their imperfect legislation. We stand up for self-government, and oppose centralization" (2.15.42). For him, neither the government nor Margaret Hale has the right to tell him what to do with the money and power he's earned.
For all of his impatience and money-hungry ways, Thornton develops a strong attraction to Margaret Hale. It might be because the stuff she says about social responsibility strikes a chord with his conscience. It might be because she takes a rock to the head when she leaps between him and a mob of angry workers. It might be because she's really pretty. In any case, we can tell from the narrator's descriptions that Thornton has his eye on Margaret. As we read in Volume 1, Chapter 10, "Mr. Thornton saw her beautiful eyes lifted to her father, full of light, half-laughter and half-love, as this bit of pantomime went on between the two, unobserved, as they fancied, by any" (1.10.2).
Thornton soon realizes what we (and the narrator) have known all along—that he's in love with Margaret. In the most classic of ways, Margaret completes him; she's his other half. D'awwww. He is practical and she's idealistic; he believes in the individual and she believes in the community. Even when Margaret rejects him, Thornton vows to love her forever, saying, "I have never loved any woman before: my life has been too busy, my thoughts too much absorbed with other things. Now I love, and will love" (1.24.26). In short, Margaret is the only woman who has ever made him stop thinking about money.
Over time, Margaret Hale's influence makes Thornton think twice about the way he treats his workers. For example, Thornton would usually want nothing to do with a labor leader like Nicholas Higgins. But once he reflects on the things Margaret has said to him, he reconsiders his opinion of the man. He especially admires how long Higgins waits outside his house to speak with him about getting a job: "Five hours […] it's a long time for a man to wait, doing nothing but first hoping and then fearing" (2.13.113).
By the end of the novel, Thornton becomes downright civic-minded in how he relates to his workers. For starters, he creates a communal fund and builds a massive dining room onto his factory in order to provide his workers with dinner. He also eats with them. As he says at one point, "Nothing like the act of eating for equalising men. Dying is nothing to it" (2.17.71). Thornton justifies these changes to his business friends by saying, "We should understand each other better, and I'll venture to say we should like each other more" (2.26.29).
It's ultimately this shift in his personality that makes Mr. Thornton a suitable husband for Margaret. That, plus Margaret has become humble enough to let herself see all of the qualities that have made Mr. Thornton a decent dude all along.