Our deepest need is to belong and to be in relationship. We need to be connected. We were built for relationship. No man is an island. In raising our children, we want them to have a secure attachment to us. If children are raised in a home with secure attachments they feel loved and deserving of love. With insecure attachments they feel unsure about these things.
When conflict develops, people that are insecure have a choice: 1) They can set limits but they face the risk losing that relationship and suffer the fear of abandonment or 2) Choose to not set limits and remain a prisoner to the wishes of another...so they passively comply but inwardly resent.
A healthy, secure three year old has developed and mastered three tasks: 1) the ability to be emotionally attached to others, 2) the ability to say appropriate "no's" without fear of loss of love and 3) the ability to take appropriate "no's" from others without withdrawing emotionally. These three tasks are expanded a second time in the adolescent years and a third time in the early adulthood. A child needs to feel safe enough to say "no". This builds appropriate boundaries. Parents need to learn to not withdraw emotionally from a child who says "no", but to stay connected. By allowing for appropriate "no's", you are teaching your child how to accept "no's" from you. If a child fears a parent's withdrawing of love when they say "no", they will outwardly comply or obey but defy inwardly.
Parental hostility can create problems in both the saying and the hearing of "no". Over-controlling parents can create dependent, enmeshed or boundary-less children. Permissive parents or ones that lack limits create entitled or spoiled children (temper tantrums).
Alcoholic families often exhibit inconsistent limits. Alcoholism causes massive boundary confusion. Adult children of alcoholics don't feel safe in relationships. Trauma experienced by children such as emotional, physical or sexual abuse or accidents, debilitating illness, death or divorce of parents can affect boundary development. Trauma upsets two necessary foundations to a child's growth: 1) the world is a reasonably safe place and 2) they have control over their lives.
Our own character, personality and selfishness can contribute to our boundary issues.