Why does smith think that pursuing personal interests is a more effective way to benefit society than trying to pursue the interest of society as a whole?

From some points I read, it seems that his argument is that people are more out to nurture their hearts with a small business and it's not always about the daily gains or profit. He also makes another point that a small business can heavily affect the local the culture. Another writer John Ikred also notes the time of this writing was in 1700 when small family businesses were more prominent and larger corporations were not, which was another influence towards his thinking.