Adding marriage onto a relationship is like adding "esprit de corps" to a friendship. It's just one more level to the relationship. It's hard to define sometimes, and difficult to explain where the depth of commitment comes from.
Think about your commitment to your childhood best friend growing up: he was your best buddy and you'd do "anything" for him. Compare that to the commitment to fellow Marines. Where you'd really do anything for them.
Marriage can (or at least should) do the same thing for a relationship. When those difficult times come (and they always will), that extra level of personal commitment that marriage brings to the table is somehow like the commitment that you have to fellow Marines in combat.
When things are really bad, that extra level of personal commitment is what can help carry you through when you really don't want to. You will not only go the extra mile, but carry the other person while you do....
[/goofy metaphor]