Is the Guru unconditional love, compassion and kindness towards us until we achieve the enlightenment. All these high lamas choose to reincarnate and going thru all the process of learning and hardship of life over and over again so that they could lead us all towards enlightenment. This is not an easy task and not easy at all, could be much harder for themselves to gain enlighten.
And in return, our Guru did not expect or ask anything in return besides asking us taking our spiritual practices serious. This is only thing they request us to do. So, always remember our Guru's kindness by doing what he have ask of you.