Couple Friends

As you get old, it becomes harder and harder to make new friends. If you are a married person, you form this new set of "couple friends" where you and your spouse are friends with another couple. In my own experience, it is harder to develop deep friendship with couple friends as compared to you being friends with another person alone.

Here are few reasons why I think this is so. First, we need some notations. Let's denote one set of partners by x1, y1 and let's denote another set by x2, y2. To make comparison with the other scenario where one person is friends with another person, let's denote the two friends here by x and y.

1. Increase in number of variables: The simple reason that number of variables has doubled makes this case complicated. Previously, it is only x and y who need to be friends with each other. Now, four people, x1, x2, y1 and y2, all need to be friends with one another, at least to a certain degree, for the couple friendship to survive. 
2. Hidden variables: Some variables that are not easily visible also come into picture. One example is the relationship variable R(x2, y2) which represents the way in which the couple (x2, y2) interacts between themselves in public. If I am to be friends with another couple (x2, y2), I also need to be comfortable about their portrayal of their relationship in public. You may not believe it, but this variable also plays its own part in deciding the strength of couple friendship.
3. Couple's stage: The stage at which the two couples are, also decides the extent to which they will be friends. If one of the couple is planning a baby or has a baby where as the other doesn't, they will have different preferences for types of activities they want to do or the amount of free time they have at hand. 
4. All other factors which decide the fate of a friendship, such as compatibility in personalities and nature, common hobbies or interests, or common habitat are obviously applicable here as well.