The Twelve Links of Interdependent Arising outlines the manner in which rebirth occurs, and the reason why living beings experience birth and death time and time again.

(1) Ignorance, of the true nature of reality, causes us to continuously engage in (2) Activities such as moral or immoral actions. These actions accumulate as karmic seeds and habits.  This karma in turn causes (3) Rebirth.  Now, a sentient being's consciousness arises once again in another body. The sentient being possesses a (4) Mind that differentiates matter as being external objects.  This differentiation based on the individual's perception gives rise to the (5) Six senses, which are the mind, sight, sound, taste, touch and smell.  Once the senses are intact and functioning, the sentient being comes into (6) Contact with external phenomena. Contact with external objects generates (7) Feelings of like or dislike.  These feelings create (8) Craving for certain phenomena. Craving leads to (9) Attachment to pleasing phenomena. (10) Once a sentient being desires certain phenomena, they perform actions in an attempt to obtain their desire.  Such actions accumulate karmic seeds and habits which then will lead to, in the future, (11) Birth and (12) Death.

Once birth (and death) occurs again, a sentient being will once again cycle through these twelve links (starting with ignorance, and so on).