Note, this is just personal opinion.
I believe, that if two people are meant to be together, life will try to tear them apart and they will lean on each other for strength and be able to weather the storm. If they are meant to be together, then no matter how much time passes, or things change, they will still look back and wonder if things could've been different. If they are meant to be, I think that even if the two people are driven apart, they will return to each other to try again, as many times as it takes for both of them to be ready for a real relationship between the two of them. If they are meant to be, you'll just know. If they are meant to be, the little things about each other that bother you will become things you love about each other in time. If they are meant to be together, the relationship won't get old, comfortable only, and stagnant.
But again, that's just what I personally believe. What you believe is what you believe. And just because we love someone more than the sun and moon, doesn't mean that if the relationship falls apart, that we will not find someone who completes our world and our soul. As they say, there are always other fish in the sea, the people you fall for before you find the one (assuming you have a one and are meant to meet them in this life) are there so that you are ready for the one when you find each other.
Listen to "Bless the Broken Road" by Marcus Hummon if you have time.