Our only reason for being here on the earth is to learn lessons, these include Love, Kindness, Understanding, Patience, Humility, etc. each one of these lessons is large and complex. Learning humility, for example, would take many years of being humiliated, being stepped on, laughed at, etc. until the person learning the lesson is brought to understand what it feels like being on the receiving end of someone's superiority or lack of compassion for others. He would learn how it makes someone feel to be down-trodden all the time. He will learn to treat others with respect and HUMILITY.
Being so complex it would be impossible to learn many such lessons in one short lifetime. It would have to be achieved over over several lives. It could be argued that the lessons could be learnt concurrently. We could after all learn several of them at the same time. Not so. Lessons are learnt by the one learning the lesson going through hardships. The harder the life - the better the lesson. If we were to learn Love - we would have to be neglected, unloved and continually cast aside by others. this is a difficult thing in ones life to bear. if they were to suffer that as well as continually being mistreated, abused, neglected, etc. in order for the lesson of kindness to be learnt - after several years life would become unbearable.
One lesson at a time would suffice, else it would be cruel. it would cause people to take their own lives for being unable to bear that 'punishment' any longer. This would make that life, that incarnation meaningless and wasted.
Understanding this principle it is clear that several lives would be required to learn all the lessons needed.
