Why is it we cling to our existence in samsara, but not to the experience in a dream?

Because a dream is very short. Upon waking up, we quickly come to the realization that the circumstances in the dream are unreal and do not exist. In contrast, samsara lasts a long time, the end is not yet in sight. When the bodhisattvas, who were once ordinary people, attain the first bhumi, it is like wakening up from a dream; they instantly realize all is an illusion, like a dream.

- Quote from The Logic of Emptiness,"Seeing All External Objects as Phenomena of the Mind"