Why Some Relationships are Overwhelming?

When a child gets born, mother and child they are such a close oneness, they are so dissolved one into another, they are like one person.
And we all need this experience in our life to be able to grow up. Without this we will be looking for it in the people around us.
For example, without this experience we will be trying to get dissolved in our spouse, which is very overwhelming for him/her.
Will such a relationship be happy, successful?