Doesn’t a marriage only need two persons? Do I mean that the support of an in-law is required?
No, what I mean is that three personalities are required for a good marriage: you, me, and us.
Consider a couple living in a small apartment. One likes to read, one likes to watch TV, and both like symphony concerts. So sometimes the TV watcher should turn off the TV. Sometimes the reader should ignore the TV. And sometimes they both go to the concert together.
In other words, a good marriage needs recognition of separate interests and needs fulfillment of shared interests.