For me it's two things.

Firstly marrying someone with whom you share core values. You have to know your core values and don't compromise. If they don't share your values don't marry them. When you share your values you don't waste time on the trivia and you treat each other in a mutually respectful way and agree on the big things like money and acceptable behaviour and what matters in your lives.

Secondly understanding that love is a verb and not just a feeling. This means actively loving the other person. It means looking for ways to make your life together enjoyable and fun. It means seeing the good in your partner, and recognising it's not their job to make you happy. It means taking pleasure in their happiness. It means appreciating them.