No sentient being is in samsara voluntarily other than certain bodhisattvas. Although some people, after being hypnotized, claim that they purposely took rebirth in samsara, it may just be a lie or their imagination. The fact is that none of us is here by choice. Why do we keep coming back to samsara? It is due to all the defilements caused by karmic power. Like growing crops, seeds do not plan what kind of fruit to yield, or any at all. Given the right temperature and humidity, fruit will grow naturally. Similarly, when people are in the bardo state, they just aimlessly drift around, not knowing specifically what to do. Most of them end up taking rebirth, as the cause for rebirth has long been committed. And with all the necessary conditions fall into place, they have no choice but to be reborn again. If one could choose, why would anyone choose to be reborn as an ox or a horse or to live in misery?

- Quote from The Right View, "The Four Noble Truths—the Path Out of Samsara"