No words necessary.
The characters don't need to exchange proclamations of love. It should be apparent by how they behave around each other. Simple things like his fingers at the small of her back. Her eyes following him across the room. Him noticing the way she rubs her knuckles when she's nervous. His smiles leaving her helplessly smiling in return.
Or, depending on the book, more dire things like her rushing into danger to save him, or him standing up to insurmountable odds to protect her. But these are things that should happen at the end, after the two have spent 300 pages or so getting to know each other.
There's more to character development than the progression of a romantic relationship.
One of the biggest mistakes I've seen writers make is introducing the love interest too early. For me at least, I have to care about the characters as individuals before I can care about them as a couple. If I'm introduced to character A, who then immediately decides s/he likes character B and sets out to make something of it, then I'm left wondering who these people are outside of their attraction to each other. When the romance is the only thing that defines the characters, then I find it very difficult to care or like either of them.
Now, of course, this isn't always the case. Love interests can sometimes be introduced in the first chapter and, in addition to being attracted to each other, they're still complete individuals with their own issues and character arcs.
And then there's the other side of the spectrum. The two are so subtle, barely ever thinking about each other outside of their own problems, that when the romance happens, the readers are left reeling and flipping back through the book to find some indication that they were meant to be attracted to each other.
(Not to be mistaken with the rivalship, in which the chemistry is still very much there, just on a competitive or unexpected level)
Some of my favorite YA romances:
• Anna and Étienne (Anna and the French Kiss)
• Grace and Sam (Shiver)
• Amy and Elder (Across the Universe)
• Kaye and Roiben (Tithe + Ironside)
So what do you guys think? What makes an effective romance for you? What are some of your favorite romantic titles (YA or otherwise)?