This is a biological, neurological, and psychological fact. This also indicates the importance of the factors present in the early environment, because any later organization will necessarily proceed from those. After a certain short initial period, the most important factor in the environment of any particular individual is that very same individual, which is to say, his or her mental structure. That is, the factors present in the child's early environment are more important than the factors of later environments. This also indicates the importance of the presence of siblings in that environment, and their emotional attitude towards their new brother or sister.
All children are egocentric.
This means that, for each particular child the center of the universe is that very same child, and his or her most important goal is his or her own satisfaction. Erich Fromm, the famous psychologist, considers that this initial egocentric stage lasts at least through age eight. This doesn't mean that children cannot pretend or imitate unselfish behavior, which they will do to obtain their parents' or caregiver's approval. And in fact, it is these early trials at unselfish behavior that create the bases for true cooperative behavior later in life. Every child will tend towards self-individuation.