Bioshock Infinite: Burial at Sea - Episode Two -
On the other side of the vent you can drop down into another room with a projector. Go ahead and watch it. The video implies that imprinting was a simple matter of human nature. Once the Songbird was hurt and Elizabeth was there to help it get fixed, imprinting was successful- they created a bond with one another. Much like the old fable about the mouse and the lion.