They don't check their love tank to see if it is getting low, not questioning if they are still communicating, or being open and honest with each other. Somehow they just expect the relationship to keep going with no preventative maintenance. They seem to be too busy to invest anything into the relationship. They never stopped to question where their relationship was at, nor heeded the warning signs that had said for years that something was going wrong. We both know that this doesn't make any sense, but couples in marriages think this way.
Every marriage needs a tune up now and then.
There are many ways a couple can invest in their marriage. Life is so busy that we need to set aside time for each other. Read books on marriage relationships together. Take time to attend a marriage seminar or retreat to refresh your marriage. Plan a weekend away, just the two of you to enjoy each other.