Family health
One of the main issues of this book is Stoner's perception of family. When he goes to college, he basically feels like he was pressured into farming by his family, and of course he wants to do well by his family, so he does whatever they tell him he has to do. But when he meets a professor who shows him that through literature, he can be part of a different family—an academic family built on a shared love for books—he can't resist it.
But, Stoner's family issue is still not solved. He finds that he is lonely for a spouse, and he turns to Edith to solve that problem. By the end of their awkward, disconnected, stand-offish relationship, he ends up hating her and falling in love with Katherine Driscoll. Edith is not family to Stoner, he discovers. He realizes that although he used to have a huge crush on Edith, the truth is, she isn't loving whatsoever.
Individual mental health
Throughout the book, Stoner suffers a horrible marriage to a bitter, resentful woman who doesn't appreciate the marriage. Stoner's wife, Edith, also abuses their only child, abuse which is the product of extreme jealousy.
What Stoner doesn't notice is that Edith is suffering from extreme mental anguish, and probably has always had paranoia and depression, but that means that the novel depicts the spread of mental health troubles, when they go untreated. Because mental health goes overlooked, Stoner cheats on his wife instead of helping her, and Edith spreads her misery outward into her family.
The need for love
Ultimately, the novel points to a need for love. The truth of the matter seems to be that Edith doesn't really like Stoner all that much, and whether he loves her or not depends on how she makes him feel, so the case could be made that Stoner and Edith are just not compatible.
So why do they stay together for so long and make a family? Because they are each subject to their real need for love. When they meet, they both need the other person, but neither wants them, so the relationship is done begrudgingly.
The saddest part of the novel shows Stoner's only daughter, having been abused by her mother, having had her relationship to her father sabotaged, and then losing her husband in WWII. This shows that Edith's failure to provide love for Grace made Grace's life a living hell.
