True colors can't hide on vacation
Unless you book separate rooms, traveling with your S.O. basically means spending 24 hours a day together. That, paired with the unpredictable factors of any trip, means there's no place to hide. You're about to learn quickly whether your boo can read a situation, problem-solve, and handle his or her shit in any circumstance.
If they can, this person is a keeper.
Communication will be tried and tested
Communication is essential to the health of any relationship. And nowhere will these skills be tested quite like while traveling together. If you can talk through disagreements while far away from what you know and your daily routine, you are on solid footing as a couple.
Traveling together forces you to either accept and work within the context of another person's limitations and expectations, or come to terms with being unable to handle them.