Without understanding samsara, realizing Nibbana is impossible. Buddha said it is because of not understanding the endless cycle of birth and death, beings continue to wander. When Buddha was enlightened, he uttered:
Through many a birth in samsara
have I wandered in vain,
seeking in the builder of this house (of life).
Repeated birth is indeed suffering! (Dhp. 153)
O house-builder, you are seen!
You will not build this house again.
For your rafters are broken and your ridgepole shattered
My mind has reached the Unconditioned;
I have attained the destruction of craving. (Dhp. 154)
The house builder of this samsara is our own craving or attachments to sensual pleasure, existence and non-existence. Once, they are abandoned, one reached the cessation of Dukkha, Nibbana.
