Well, if you want it to last, you have to be prepared to weather the storms. No one is sweetness and light 24/7 for 60 years. So you need to work at it to restore respect, love, affection and compassion during and after childbirth, job loss, ill health, family tragedies, children's issues, thwarted dreams and goals, preoccupation or workaholism. Inevitably there are times when you fall out of passion and lust for your spouse and they don't always coincide with their loss of interest in you. There's over familiarity too - you have heard all his best jokes, anecdotes etc in the first five years. By year 25, you are bored to tears of his best dinner party stories and can see in his eyes that he feels the same about yours. Snoring or nose blowing or leaving a cup beside the dishwasher is a little annoying at first. 30 years later it feels like grounds for divorce!
For marriage to work, both people have to really want it to work, to share the physical, emotional, financial burdens equally or divide them in a way that genuinely suits both people. You have to work at retaining fun and adventure and humour and affection and respect. You have to learn to be tolerant if they are ill or lose their job or descend into depression, and navigate through, deciding whether it's in both people's interest to stay or split.
When life is easy - no kids, small mortgage on a flat, loads of free time, it's so easy to be in love. Harder when a child has puked all over you for the fourth night in a row and the washing machine has broken down and your partner's been let go from work for the third time in five years but he's merrily snoring away while you handwash baby, self, sheets, clothes at 3am and wonder if you'll ever have fun again.
You have to adjust to change - one person becoming more successful or fitter, or less fit or less powerful in the world. The things you loved and admired at first may not last so you have to work out what else is worthy of the long haul.