Theory of Moral Sentiments in many ways is a book of social and moral psychology. What we can call economic behaviour is necessarily situated in a moral context; it does not exist in some separate sphere that somehow isolates it from the moral norms of society at large. But more than that, the key theme of the book is an opposition to the view that all morality or virtue is reducible to self-interest. The moral interactions Smith treats in Theory of Moral Sentiments bear on the practices that characterise his contemporary commercial society.
The very complexity of that society, with its extensive division of labour, means that the bulk of inter-personal dealings are with strangers. These dealings are conducted on the basis of adhering to the general rules of justice, rather than the necessarily particular basis of mutual love and affection. Beneficence is thus less essential to the existence of society than justice.
Nothing in this means that Smith is denying the virtuousness of benevolence. When Smith came to write The Wealth of Nations he made it clear that the ‘wealth’ lay in the well-being of the people. This covered not only their material prosperity, being better fed, clothed and housed, but also their moral welfare. Accordingly he thought to be in poverty is to be in a miserable condition and also that to be condemned to repetitive limited tasks (like sharpening pins several thousand times a day) damaged our ‘social’ and ‘intellectual virtues’