Let's face it. John Thornton can sometimes be a bit of a mamma's boy. And how can you blame him, seeing as how his mother takes an interest in every aspect of his life? First on her list of priorities is making sure that no girls try to marry John and take him away from her. When John first meets Margaret Hale, Mrs. Thornton jumps all over him, saying, "Take care you don't get caught by a penniless girl, John" (1.9.26). She gets even angrier when she finds out that Margaret Hale thinks she's too good for John, adding, "What business had she, a renegade clergyman's daughter, to turn up her nose at you!" (1.9.32).
Okay, so here's what we know. Mrs. Thornton loves and respects her son for pulling himself (and his family) out of poverty through his hard work and intelligence. She also wants the whole town of Milton to respect him. As the narrator tells us directly at one point, "she looked fixedly at vacancy; a series of visions passing before her, in all of which her son was the principal, the sole object—her son, her pride, her property" (2.1.5).
So yeah, Mrs. Thornton has an attachment to her son that some people would call unhealthy. On top of that, she is a very shrewd and cold woman who can be tough to get to know. We learn this about her early on: "Mrs. Thornton was not a woman much given to reasoning; her quick judgment and firm resolution served her in good stead of any long arguments and discussions with herself" (1.12.10). In other words, Mrs. Thornton has hard opinions on things and she doesn't waffle. Heck, she doesn't even think about why she has opinions. The woman just knows what she likes and what she doesn't.