One of the best parts about marriage also tends to be one of the most significantly impaired. While no conclusive research has come out on these top three, a general assumption among marriage therapists is sex, money, and conversation are the biggest failure points for a marriage.
It would make sense that communication between two people are the most important. We even see this in our faith, right? When we struggle with our relationship with God, what suffers first? Attendance at church and worship? Prayer? Reading Scriptures? These are all ways we commune with God. Sin blocks our communication, whether with God or the one we love the most.