Aside from divorce I believe the biggest fear most people have is their feelings will change or their love and passion will lessen for one another overtime. We hope everything will remain the same.
The reality is nothing and no one goes unchanged overtime. It's natural to prefer a "known present" over an "unknown future".
There will always be unforeseen circumstances and events. There may even come a time where you no longer want the same things out of life and think it best go your separate ways. No one thinks about divorce on their wedding day and yet 50% of marriages end with divorce. Another percentage of couples stay "unhappily married", live apart but never file for divorce, have sexless marriages, or settle into something resembling roommates with the same last name once they have an empty nest.
Marriage does not guarantee a lifelong commitment of love, passion, and happiness until death do you part. It's not like a sofa you purchase and sit it in a corner, walk away, and it will be fine for the next 30 years. Marriage is more like a garden which requires constant nurturing and if neglected it will surly die. However the biggest challenge is selecting the "right" mate for yourself to begin with.
You want to make sure you share the same values, want the same things for the marriage, naturally agree on how to obtain those things, and last but not least have a mutual depth of love and desire for one another.
Commitment does not mean there are no "deal breakers". Commitment means we do our best to resolve our issues before they become "deal breakers".
There is no amount of "work" or "communication" that can overcome being with someone who does not want what you want. We're either growing together or growing apart. There is no neutral.