This question becomes important for adoptive families, because some adopted children will form an almost immediate dependency bond to their adoptive parents. To mistake this as secure and healthy attachment can lead to many problems down the road. Attachment is a process. It takes time. The key to its formation is trust, and trust becomes secure only after repeated testing.
Normal attachment takes a couple of years of cycling through mutually positive interactions.
The child learns that he is loved, and can love in return. The parents give love, and learn that the child loves them. These must be consistently present for an extended period of time for healthy, secure attachment to take place. It is through these elements that a child learns how to love and how to accept love. Older adopted children need time to make adjustments to their new surroundings.