From Ask Dr Wiki
Relationship are established by on-going communication. Whether it is friendly, hostile or technical it causes long term changes in both brains, which include memories of the relationship and expectations for its future. These changes result in “Emotional Bonds” between the two communicating parties.
There is no conscious awareness of the emotional bond as long as the intensity of the relationship remains the same or increase. If it decreases the the two begin to miss each other increasingly as time passes. This motivates them to increase communication, if possible.
The strength of an emotional bond is proportional the total amount of communication that can be remember both consciously and unconsciously. Amount includes the total number of bits, and associated emotion, and importance attributed to the information, and fantasy communication.
It is possible that one party develops a stronger bond than the other due to a greater amount of fantasy or greater attribution of importance to what transpires.
If a person to which we have a bond dies we experience a sudden severe emotional pain, the intensity of which is proportional to the strength of the bond.
If communication decreases or stops the bond gradually fades away. This may occur if two bonded people are separated long enough by circumstances or if a decrease in communication is so gradual it isn’t noticed.
Normally, a mother is well bonded to her infant at birth because kicks count as communication and she may have carried on a fantasy relationship even before conception.
Communicative bonding occurs in higher animals as well. It is a gradual process, different from just meeting someone and liking them, and from the imprinting of baby ducks.