You learn if you can truly get along as a couple when you live under the same roof, and have to communicate and share responsibilities. That is when you learn if you can communicate... you learn how your partner is. All those habits and quirks. If you can really get along. What annoys you, and what doesn't. But most of all, you learn about yourself. How you handle that situation, where your flaws and strengths are. What kind of person you are attracted to... maybe even who you are at that point in time.