They’ve been through challenges
At a certain point, a couple may have faced a tremendous amount of challenges together. There may have been sickness, loss of jobs, loss of loved ones, collapse of businesses, and who knows what other catastrophes life threw at them. When a couple makes their first vows, they’re promising to be there for each other through hypothetical sickness, poverty, and such. So if they are actually there for each other through those things, that deserves a celebration.