Happy nature and the opposite

[D1W1]

Why do some people have happy nature, other people have sad nature? No matter how diligent they try they always get back to their default state. What's the answer to this?

[SarathW]

According to Abhidhamma beings are born according to their past Kamma (Ahetuka Vipaka Citta)
However, using Citta Niyama they can change their ways with a lot of effort.

[Mr. Seek]

Different lives, different ups and dows. Different conditioning.

[Ontheway]

According to Abhidhamma beings are born according to their past Kamma (Ahetuka Vipaka Citta)
However, using Citta Niyama they can change their ways with a lot of effort.

From Abhidhamma bhajaniya method, not all beings are born according to Ahetuka Vipaka Citta.

There are 19 types of consciousness, to which they served as bhavanga, Cuti Vinnana, and Patisandhi Citta (rebirth-linking consciousness or birth consciousness; in Suttanta method these were called birth consciousness, Abhidhamma method called them as rebirth-linking consciousness, they should be treated as the same). By these consciousnesses, beings are bound to samsara and reborn accordingly as taught in Bhava Sutta.

"Herein, rebirth-linking by means of the unprofitable-resultant root-causeless mind-consciousness element comes about in the states of loss.

Rebirth-linking by means of the profitable-resultant comes about in the human world among those blind from birth, born deaf, born mad, born drivelling, the sexless, and so on. Rebirth-linking by means of the eight principal resultant consciousnesses with root-cause comes about among deities of the sense sphere and the meritorious among men. That by means of the five fine-material resultant kinds comes about in the fine-material Brahma-world. That by means of the four immaterial-sphere resultant kinds comes about in the immaterial world. So rebirth-linking [consciousness] conforms to the means by which, and the place in which, it comes about."

- Visuddhi Magga, Chapter XVII


Therefore,
1 Citta (bound for woeful planes of existence)
1 Citta (born among the unfortunates such as blind, mad, sexless, etc. in human realm)
8 Cittas (born among meritorious human and heavenly beings of six sense-sphere heavens or six Kamavacara deva lokas)
5 Cittas (born in Fine Material Brahma worlds or Rupa Brahma Lokas)
4 Cittas (born in Immaterial Brahma world or Arupabrahma lokas)

Adding all together, these 19 consciousnesses served as the "seed" as described by the Buddha that "germinate" in different realms and further "grow" accordingly.