How do you define love? And to be clear, I’m talking about the love two people feel towards each other, not the kind of love we apply to, say, watermelon (although my love for watermelon is indeed profound).

Dictionary.com has several definitions for love, but the one that comes closest to what I’m talking about says love is “a profoundly tender, passionate affection for another person.”

Does that cover it for you? It doesn’t for me. Not by a long shot. I think love runs deeper than that.

The...