This is because what's beneath the romantic love is a genuine friendship, a deep love and genuine respect that almost comes naturally with actions (not all the time, but over the span of time) and that the other person feels and can trust. Most marriages would probably last if the friendship were there, but too many marriages are based on a more surface-level love that rarely lasts and that is likely entered into because "I'm X age / We've been together X long so it's time to get married." - From, a 2x divorced & now-forever-married woman