Over the years of working with people going through a divorce, I've heard repeatedly that one of the parties was blindsided by the other spouse's request for a divorce. With that said, a relationship headed for an impending divorce will often show multiple signs of challenges and issues, if you're paying attention.
One of the most frequent complaints is that love has faded. Know that it's normal for relationships to change over time. Your love will also change. The first feeling of romance will turn into a deeper kind of love. However, it shouldn't disappear. A relationship that has lost its love altogether is in danger.
Anger and resentment can do immense damage to a relationship. If you both resent each other, then it's a strong sign the marriage is in trouble.