With titles, though, you encompass all of your story's elements into a few words. Not all writers try to do this; for younger audiences, the titles might be more concrete (The Berenstein Bears And Too Much Vacation), but as you go into the scholarly set, people want clever titles with meaning.
Most often, the title centers on the object mentioned in the title, whether it's a person, or a significant object. We call this the literary MacGuffin, because we, the reader, want to find out who or what the thing mentioned in the title is. (Macguffin: object that protagonists in a story want. The literary MacGuffin is something that the readers want.) In fact, Wendelin Van Draanen wrote two Sammy Keyes books when these MacGuffins popped up as titles, and she wrote to find out what the Sisters of Mercy and a Runaway Elf were respectively. Another great example is Harry Potter and the Prisoner of Azkaban, since Sirius Black, the title prisoner, is an integral character that the whole world is searching for.
This strategy doesn't always work, however, because your literary MacGuffin might be cliched. If your book has a lot of MacGuffins, then you have to pick the one that encompasses the story the most. My whole story is about Magic Turning into Memories, but that title doesn't have the zing I'm looking for since magic is a necessary staple of fantasy novels and memories is a huge mouthful. So what is an author to do?
The solution is to make the theme sound less cliched. So I need to find apt synonyms for magic and memories respectively. Either that or use the theme of Midsummer and Snow, or something off about the woods, since all of these appear in the book. Hey, it might happen.