Nick Dormer wants to pursue a career in painting instead of his family's traditional role in British politics. This upsets his family and particularly his lady friend, Julia Dallow, a beautiful but demanding woman deeply involved in political campaigns. But Nick's old Oxford friend Gabriel Nash encourages him to follow his desire to become an artist. Despite his misgivings Nick goes through an election campaign and wins a seat in Parliament. He proposes marriage to Julia but they agree to wait.
Meanwhile, Nick's cousin Peter Sherringham, a rising young man in the British diplomatic service, encounters a young actress, Miriam Rooth, in Paris. He falls in love with Miriam, who shows great energy but is a woefully raw talent. Peter introduces Miriam to French acting coach Madame Carre, and Miriam begins to improve her acting technique greatly.
Nick at last tires completely of politics and resigns from Parliament. He thus loses a large bequest from his political patron, Mr. Carteret. Nick becomes a full-time painter, and when Miriam comes to London in search of theatrical success, she sits to Nick for her portrait as "the tragic muse." Julia finds the two together in the studio. Although nothing improper is going on, Julia suddenly and bitterly realizes that Nick is dedicated to art and will never return to politics.
Miriam eventually triumphs as an actress, especially as Juliet. Peter proposes marriage to her, but she refuses and instead marries Basil Dashwood, her business manager. Peter accepts a diplomatic assignment in Central America. He returns to London on leave and becomes engaged to Biddy Dormer, Nick's sister. The novel ends with a suggestion that Nick and Julia may eventually marry, after all.

Question: Why is Julia upset when she finds Julia setting for a portrait for Nick?
Answer: Julia realizes that Nick really wants to be an artist and will never return to politics.
Alternative answer: because nick will never go back into politics