What do you think keeps relationships going strong long-term?
Common courtesy. The little "every day things".
"I'm going to the kitchen. Did you want another drink?"
"I'm at the market. Need anything?"
"I was out & found your favorite juice."
" you said you needed new dress shirts a couple of weeks ago so i picked you up a few "
"Thank you for letting me use your car. I topped off the tank & vac'd it out"
The cynic in me wants to say stagnation. I've know several couples who stay together because they can't imagine something different for themselves. Even though there's a preponderance of evidence pointing to the fact that one or both aren't happy. I think recognizing that the meat of the matter is in the struggle and intentionally celebrating and behaving gratefully for the moments of happiness is a good start.