What’s Up With the Ending?
In the end of life, we all die. Sorry to bring you down.
But in the end of Daisy Miller, only Daisy dies. (Kind of a shame, because we find Mrs. Costello really annoying.) Daisy's death is a tragedy not only because she's a young, beautiful girl—though, it should be noted, that's a popular kind of tragedy. (It's a revealing fact that the public seems far more upset when someone hot and female dies—think Marilyn Monroe or Princess Diana.)
When Daisy dies, a lot dies with her. Mainly, Winterbourne's hopes and dreams. He's a little bit dead inside when he meets her, but she brings him to life with her unconventional behavior, quick wit, and lust for life. Think about how she livens up everything they do together (which is basically their trip to the castle at Chillon in Part 1):
Winterbourne's preference had been that they should be conveyed to Chillon in a carriage; but she expressed a lively wish to go in the little steamer; she declared that she had a passion for steamboats. There was always such a lovely breeze upon the water, and you saw such lots of people. The sail was not long, but Winterbourne's companion found time to say a great many things. To the young man himself their little excursion was so much of an escapade—an adventure—that, even allowing for her habitual sense of freedom, he had some expectation of seeing her regard it in the same way. (1.242)
While Daisy represents innocence, she also brings out the innocence and child-like wonder in others, especially Winterbourne. He wants to go to Chillon in a carriage but she's like, "let's shake it up and take the steam boat!" That Daisy. She's so Raven!