Marriage. Everyone who is married, regardless of station in life, wishes for a loving marriage. When we fail or go through difficult times it can affect our spirits, lower our mood as our emotions connect to our brain chemistry and we start to feel low and in some cases slow intellectually. Depression and low mood can have dramatic affects on all aspects of our lives. Remember how great falling in love felt.
Influence. A marriage provides an opportunity for one person to have a great deal of influence on another. In a good marriage this mutual influence is part of what makes the marriage work (see john gottman's work). Since one's physical and intellectual development is affected by habitual behavior, a couple's relationship over time will show in the physical and intellectual functioning of the two people.