One of my happily married friends explained that it's important to keep up the playful energy in a long standing relationship. In his case, he frequently makes funny voices for their pets. Another thing they do is use really stupid random pet names. "Care for another onion ring my Onion Ring?" On a darker side, I have read a lot of testimonials online of why relationships fail and a huge cause of relationship failure is the wife getting bored and starting an affair for the excitement. One theory for why assholes seem to have sex appeal is that for all their flaws, they are not boring. Having a good sense of humor is a good way to not be boring while also not being an asshole
I think personally in a marriage it dies when you stop creating it actively, through humour or otherwise. Like many things, neglected it falls apart. I have a boundless sense of humour, childish at times and borderline offensive at others but I make my wife laugh so that's what's important. [LoR hastily blocks out all memories of his wife's eye-rolls and deep sighs from his more childish jokes]