What separates couples?

Sure, money, inlaws, illness, substances, military service, flings, politics, and much more. In our case it was differences in parenting. I was wrong. VERY wrong.
Today, a nice younger couple from New Hampsire bought my latest boat, in a fair deal to both parties. I spent the day with them teaching how to be good owners, so it will be fun and last a long time.
I think boat ownership should be added to the sad list of things coming between couples.These two were a nice match, and loved each other by all appearances. And they were good learners (of course they had a tree top teacher), and already had many technical sailing skills. And it was gratifying to see how they cooperated with each other, and with me. No kids yet, and both were educated and with good jobs.
A perfect picture. Until things started to go wrong, which of course on any watercraft, is bound to happen. Regularly. And at the worst possible times and circumstances.
She seemed better at technical things than he did, yet he seemed to try to do more than his share of tasks. He made his share of understandable errors, all of us as novices make. She and I were calm and understanding, as we all made our share of the usual goofas well.
But then I saw it, and heard it. As his frustration grew, he also began to speak a little more harshly with her. Once saying, in a slightly angry voice, "I can't believe you did that the way you.did." She kept her calm supportive demeanor.
Overall, it was actually congenial. But couples who have been there will know the worrying signs.
Here I am signalling virtue, when in fact, in younger years, I wish I was more patient.
Experientia docet.