I always thought you bond with a person when you experience things together which could be anything.
That would be part of it.
We learn by experience. i.e.
Perception determines conception, conception determines will.
Not only shared experience, but comperable mental abilities.
But this does not mean, simply staying together.
There is a difference between a heap of people, and a civil one, same is true of relationships.
To become one is a lifelong commitment to virtue.