لعلَّه يزَّكَّى (he might be purified): He might be purified from the material or moral dirt by what he hears from you of knowledge which will increase his faith. As they were newly Muslims and the community was full of wrong and bad matters, some people could not overcome even after they converted to Islam, in this Makki stage. Such as those people are in need of remembrance which drive them to the righteousness.
So that ‘O, Muhammad) your job is only to convey, so do not tire yourself nor overburden it with more than what is required.’ This is the key point to understanding the admonishment given to the Prophet of Allah (saws).
Allah (swt) had not addressed the Prophet (saws) by saying: “You frowned and you turned away.” Rather, He had addressed him in the form of the third person, so that he would not be subject to being addressed directly as the second person.
Thus, it is as if Allah (swt) is saying: ‘Look at the protectiveness of the Prophet (saws) towards the religion of Allah (swt). He has turned away from the easier path and has chosen the more difficult one.’