Although the two displayed great respect and admiration for each other, their friendship was uneasy and had some qualities of a love-hate relationship. Harold C. Schonberg believes that Chopin displayed a "tinge of jealousy and spite" towards Liszt's virtuosity on the piano, and others have also argued that he had become enchanted with Liszt's theatricality, showmanship and success. Liszt was the dedicatee of Chopin's Op. 10 Études, and his performance of them prompted the composer to write to Hiller, "I should like to rob him of the way he plays my studies." However, Chopin expressed annoyance in 1843 when Liszt performed one of his nocturnes with the addition of numerous intricate embellishments, at which Chopin remarked that he should play the music as written or not play it at all, forcing an apology. Most biographers of Chopin state that after this the two had little to do with each other, although in his letters dated as late as 1848 he still referred to him as "my friend Liszt". Some commentators point to events in the two men's romantic lives which led to a rift between them; there are claims that Liszt had displayed jealousy of his mistress Marie d'Agoult's obsession with Chopin, while others believe that Chopin had become concerned about Liszt's growing relationship with George Sand.