When they think of you before they think of themselves.

ex.

My wife recently discovered that she is allergic (violently sick allergic) to corn, wheat and dairy.

I am type 1 diabetic. Have been since I was 5. I can eat carbs, but should keep them and processed sugars to a minimum.

When my wife goes shopping she stresses over carb counts and getting me low-sugar foods. She rarely ever thinks of herself and has come home at times with absolutely nothing in the bags that she can eat.

When I go shopping I stress over finding her something without corn starch or wheat flour. I'll come home with several things that she may like, but they'll be high-sugar fruits and high-starch potato items.

It's a simple and mundane example, but it's true. We think of each other before thinking of ourselves. To me, that's true love.