When we are attached to nirvana, nirvana is samsara for it leads to rebirth in samsara.
Samsara consists of the Six Realms of Desire —
- the Hells,
- the Realm of Hungry Ghosts,
- the Realm of Animals,
- the Realm of Humans,
- the Realm of Daemons, and
- the Realm of the Gods
When one is attached to rebirth in the human realm, that is not Buddhism but Sanatanadharma and results in rebirth. This attachment consists of a strong belief in transmigration and the permanent soul that is reincarnated solely as a human. It also violates the law of karma, which allows for rebirth in the Six Realms.
In Buddhism, non-attachment is an ideal which may be achieve by purifying the mind. The difficult route is to practice No-thought i.e. let no thought attract you to follow it to its end, but first let each thought go and return to the breath. With practice, it becomes as easy as breathing. This is the essence of Zen.
As for the easy route, Buddha Recitation leads to non-attachment as the Name-that-calls purifies the mind with earnest repetition, burning evil karma as you remember the Buddha. This is why it is called Buddha Remembrance. This is the essence of Pure Land Buddhism.
By remembering the Buddha, the mind is purified so that non-attachment to Nirvana is easily assured!
For non-attachment to Nirvana is essential to becoming Unborn.