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. Why is it we do not have this capability? The reason is we are still dreaming. In the dream, we cannot discern what is real or unreal, so we continue to grasp and cling to worldly existence.

This is the view held by relatively advanced practitioners among the Yogacara True Aspectarians. In their encounter with ordinary people, however, they do not make this assertion since it cannot be comprehended by most. They acknowledge the existence of other sentient beings (so it is not Solipsism). In a group of one hundred people all looking at the moon at the same time, each will see the moon differently. But those with common karma will all have the same feeling and believe they are looking at the same moon. If one person closes his or her eyes, leaves the group or dies, that person’s perception disappears, but all others will still think the moon exists. The logic employed in the True Aspectarian view is very complete and has yet been refuted.

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