This is true of any kind of love, not just romantic. How do you show familial love? How do you show friendship so deep that love is the only word for it?
Sometimes comics makes it easy. Obviously saving each others' lives is a great demonstrator of feelings. It's not like we need to be told that Batman and Robin would die for each other, or Superman and Wonder Woman, or [insert names here].
But what happens when you can't rely on deadly peril to make the point for you? Few of us face so much deadly peril in our day-to-day lives. How do you demonstrate love?
I got started thinking about this when I read a fic in a different fandom that tried to demonstrate love through a break-up. Character A walked out, and Character B completely fell apart. And I mean completely. Stopped eating, stopped sleeping, the works. And I believe that this was meant to demonstrate love. But it didn't read as love to me. It read as need. Which is a different animal entirely.
I suppose it depends a lot on your definition of love. My favorite working definition is one I nicked from Robert Heinlein's Stranger in a Strange Land:
'Love' is that condition in which the happiness of another person is essential to your own.
Happily, this kind of love happens to be ridiculously easy to demonstrate in fiction, especially if you choose the right POV.
(Of course there is another, broader definition of the word that encompasses spite and vanity and revenge. Under that definition, even the Joker is usually motivated by love—love of life or love of death, if nothing else.)
How about you all? How do you define love, and how do you write it?