5. They go the extra mile for each other.
That's no metaphor, either. For example, when Molly gave Mike a fitness tracking, he wasn't so pumped to have his physical activity under a microscope—but that's where Carl came in handy! He was super-supportive of his partner's plight to be healthy, but he was also willing to wear the tracker, up the number of steps, and earn Mike some brownie points with his wife.