Vacations are like life. It's better if you're trapped in one with someone else.
Lounging on a hotel balcony, looking up at the blue blue sky. Falling over in the lobby after a lovely drunken evening, laughing and hushing each other before security appears. Sneaking away from the rest of the tour to find a secret spot, and curling up together to enjoy those last few precious moments before you're caught. Going shopping for things slightly out of your budget, just so you can stuff them in your luggage and take them back home.
There are millions of ways you can enjoy a single piece of luggage. Honest. Literal millions. Especially if you stop at more than one place. But they're all a little bit better if you have some extra room to share your space with someone you love. That's why a three-piece set will always be superior.