The Buddha shares on the difficulty of regaining human existence for an undiscerning person who has fallen into a lower realm.
Paṭhamachiggaḷayuga sutta - Yoke With A Hole (First)
"Bhikkhus, suppose a man were to throw a yoke with a single hole into the great ocean. And there was a blind turtle. This turtle would come to the surface once every hundred years.
What do you think, bhikkhus, would that blind turtle, coming to the surface once every hundred years, ever manage to insert its neck into that yoke with a single hole?"
"It would be very unlikely, venerable sir, that it would ever do so, only after a very long time, if at all."
"Sooner, I say, would that blind turtle, coming to the surface once every hundred years, manage to insert its neck into that yoke with a single hole than an undiscerning person who has fallen into a lower realm would regain human existence.
For what reason? Because, bhikkhus, there is no practice of the Dhamma, no wholesome conduct, no doing of wholesome actions, no performance of meritorious deeds in those lower realms. There, bhikkhus, beings devour one another, and the weaker are prey to the stronger. For what reason? Because, bhikkhus, they have not seen the Four Noble Truths. What four? The noble truth of suffering, the noble truth of the arising of suffering, the noble truth of the ending of suffering, and the noble truth of the way of practice leading to the ending of suffering.
Therefore, bhikkhus, effort should be made to fully understand: 'This is suffering'; effort should be made to fully understand: 'This is the arising of suffering'; effort should be made to fully understand: 'This is the ending of suffering'; effort should be made to fully understand: 'This is the way of practice leading to the ending of suffering.'"