Referring to "The Ones Who Walk Away from Omelas," why would a person you love walk away just so quickly?Isn't it that hard to let them go?

If this question is geared towards LeGuin's short story, there is much here that strikes at its very essence.  LeGuin constructs her narrative into two parts.  Both are connected to one another, but there are real questions as to how the first can allow the second to exist.  It is in this vein that I see your question emerge.  The idea of how one can let someone go in order to become the child locked in the dungeon is an important one.  Reflect for a moment as to its implications.  This child is locked away and is the reason why the people of Omelas can enjoy their lives.  While there is prosperity, joy, elation, and a sense of satisfaction in consciousness, this child remains in the basement, the sole reason for all of this joy that they cannot experience.  Certainly, there were parents for this child and a natural question present in examining the child's predicament is where are those child's parents?  Will that child ever experience the love so flagrantly denied?  LeGuin suggests that even if the child were to be reclaimed, its entire life is one of despondency and there could not a full reckoning of the change involved.  The second aspect of the question might be aimed to those who walk away from Omelas.  Upon seeing the condition of the child, there are individuals who simply must walk away from such an existence.  The emergence of guilt becomes too much to bear, according to LeGuin.  Remember that the one absent condition of Omelas is guilt, and upon its emergence, distance from Omelas is evident.  There must be a great deal of difficulty to let someone leave under such circumstances, yet I think that this is where one has to balance the ability for someone to live a life that has some level of happiness as opposed to allowing them to remain in Omleas, fundamentally miserable at knowing their happiness is contingent on the abuse and suffering of this child.  It is because of this that there is so much emotional ambiguity in LeGuin's work, causing us to recognize that the pain that lies at the heart of modern consciousness is the pain of being pitted between equally painful ends with no respite from such a condition.