An adult understands the lifelong emotional consequences of giving up a child, while the child's first experiences are merely of waking up to an unfamiliar world. All children have the experience of discovering things about themselves as they age, and though the pain of being abandoned might strike them deeply when they are old enough to understand it, many adopted children don't feel that they are "missing" anything because their home lives are perfectly fulfilling. The parent who gives up their child will always remember the choices that led to their decision, and those choices may continue to be a source of pain for the rest of their life.
Children who are adopted have no idea who their biological families are. This is especially true for children of closed adoptions. Not knowing who else in the family has one's eyes, ones mannerisms, etc., would be very difficult. In addition the adopted child must constantly struggle with the question of why his or her parents gave him or her up for adoption.