Lambert Strether, a middle-aged, yet not broadly experienced, man from Woollett, Massachusetts, agrees to assume a mission for his wealthy fiancĂŠe: go to Paris and rescue her son, Chad Newsome, from the clutches of a presumably wicked woman. On his journey, Strether stops in England, and there meets Maria Gostrey, an American woman who has lived in Paris for years. Her cynical wit and worldly opinions start to rattle Strether's preconceived view of the situation.
In Paris, Strether meets Chad, and is impressed by the much greater sophistication Chad seems to have gained during his years in Europe. Chad takes him to a garden party, where Strether meets Marie de Vionnet, a lovely woman of impeccable manners, separated from her reportedly unpleasant husband, and Jeanne, her exquisite daughter. Strether is confused as to whether Chad is more attracted to the mother or the daughter. At the same time, Strether, himself, feels an overwhelming attraction to Marie de Vionnet, which he suspects she might requite, and so begins questioning his commitment to return to Woollett and marry Chad's mother, despite his admiration for her.
All of these impressions of Parisian culture lead Strether to confide in Little Bilham, a friend of Chad's, that he might have missed the best life has to offer; he starts to delight in the loveliness of Paris, and stops Chad from returning to America. Strether's American traveling companion, Waymarsh, provides thematic counterpoint, by refusing to be seduced by the charms of Europe. Meanwhile, Mrs. Newsome, Strether's fiancĂŠe and Chad's mother, impatiently waiting in America, enlists new "ambassadors" to return forthwith with Chad. The most important of the new ambassadors, Sarah Pocock, Chad's sister, harshly dismisses Strether's impression that Chad has improved, condemns Marie as an indecent woman, and demands that Chad immediately return to the family business in America.
To escape his troubles, Strether takes a brief tour of the French countryside, and accidentally encounters Chad and Marie at a rural inn; he then comprehends the full extent of their romance. After returning to Paris, he counsels Chad not to leave Marie; but Strether finds he is now uncomfortable in Europe. In the end, he declines Maria Gostrey's virtual marriage proposal and returns to America.

Question: Why does Strether tell Chad not to return to America?
Answer: He realized that Chad was in a relationship with Marie and wanted them to be happy.
Alternative answer: Strether does not want Chad to leave Marie.