I have found that middle children typically will throw their first-born spouses a bone once in a while without letting them know how they really feel.
Some practical suggestions for first borns married to middles include: Born Plus Last Born Equals Bliss (Usually) According to one study of three thousand families, the odds for a happy marriage increase a great deal when the first born hooks up with the last born.
What is at work here is the opposites-attract-and-are-good-for-each-other factor.
The first born teaches the last born little things that may be lacking, such as being organized and having goals, while the last born helps the first born lighten up and not take an overly serious approach to life.
For example, she taught me that girls don't like being approached by a bunch of boys who are show-offs--pushing each other, talking loudly, and doing stupid things that guys often do.