Even Buddha didn't clearly mention "Root Cause of Ignorance". One morning, Root Cause of Ignorance flashed into my mind and I could clarify it unexpectedly.
"Ignorance" means not understanding the ultimate truth of "Interdependence" (emptiness). The opposite of ignorance is "Wisdom" which means understanding the ultimate truth of "Interdependence" (emptiness). Buddha preached that ignorance is the source of all sufferings and the wisdom is the source of all happiness. So, his teaching was to attain the wisdom to transform sufferings and to prevent sufferings.
The reason why this wisdom is vital is because ignorance causes "distorted view (ego)" that is "I am special, separate, superior to others", which separates oneself from others and generates "wrong perception". Meanwhile, wisdom brings "correct view (true self)" that is "We are all the same fundamentally", which is inclusiveness and enables us to attain "right perception".
However, I am convinced through my actual experience that even if I attain the wisdom, I can't practice compassion (love others) as long as self-love (self-acceptance) is lacking in me. This means that wisdom only can't be the method to transform sufferings and prevent sufferings. You can easily guess this from the fact that even H.H. the 14th Dalai Lama couldn't understand "self-hatred". (Please refer to the following)
Buddha taught that every thing and phenomenon is interdependent, so he must have understood that ignorance also has its cause. I am not sure whether he didn't mention even though he knew it or because he didn't know it. But I could clearly see through the root cause of ignorance in this occasion. If Buddha didn't mention even though he knew it, I guess that he kept silent out of his compassion because he understood deeply that sufferings and happiness are interdependent and that if sufferings are removed, happiness is also removed.
I am convinced that the root cause of ignorance is due to lack of unconditional love from parents in babyhood. Children who were not accepted unconditionally (not given unconditional love) by their parents, can't love themselves. Sometimes they hate themselves. (Irrespective of whether consciously or unconsciously, even if parents acted for the happiness of their children, the children may not welcome it most cases.)
Children are frightened at their parents of fear and insecurity if they are not accepted unconditionally. Therefore, it is necessary for them to make up their ego that is a distorted view of "I am special, separate and superior to others" to protect themselves from parents. Ego separates oneself from others, so children are not aware that their ego doesn't protect them and that rather causes sufferings.
If s/he has ego (distorted view or fake self), s/he can't love others as a matter of course. Sometimes s/he hates others. Those who can't love oneself can't love others. Far from it, those who have ego take self-centered attitudes.
If s/he takes self-centered attitudes, s/he won't be loved by others as a matter of course. Sometimes s/he is hated by others. People rarely can love those who don't love them. Usually, it's hard to love those who don't love them.
As a result, these children suffer. People suffer if just the way they are are refused and hated by others. This environment doesn't help them to nurture self-love. It rather helps them to develop self-hatred and to enter into vicious cycle.
My understanding on "Unconditional Love" is the following.