Conventionally I understand that people think that when a human is born it is the product of the parents, partly true, but is some part of it also reincarnation of some other self?
The parents provide the gross physical body for the pleasure and pain of the subtle body of a Jiva. Brahma Sutra Bhasya of Sankaracharya 3.1.27 says,
From the womb a (new) body is born.
It is only when the soul (Atman and the subtle body) reaches the womb from candraloka that it gets, according to its residual Karma, a body for the enjoyments of pleasure and pain. In previous stages, it only gets connected with ether, air etc (during its journey from candraloka to the womb after the exhaustion of its karma).
Hindu religious literature has a numerous references to reincarnation. In The
Bhagavad Gita, Sri Krishna, a Divine Incarnation, says to his student Arjuna,
Arjuna, both you and I were born many times in the past. You do not remember those births, but I remember them all.
In this particular context Sri Krishna can be called a Jatismara, a person who remembers his past births—but Arjuna is not.
•Why Do We Reincarnate?
Hinduism says that our unfulfilled desires are primarily responsible for our rebirth. To understand this position one should know Hinduism’s view about death and thereafter.
•Unfulfilled Desire Causes Rebirth When people die with strong unfulfilled desires, which can only be fulfilled on earth, their minds—while they are in the other world—strongly yearn for the fulfillment of those desires. As every conscious action is prompted by a thought, those unfulfilled desires eventually bring them back to earth, thus causing their rebirth or reincarnation.
By Swami Bhaskarananda
Human or any jiva gets born according to his previous karmas and desires.
When favourable condition meets according to his karma and desire he may take birth into that family.
Eg. If a person has borned in poor family. He does good karma throuout his life and wishes to be prosperous. But somehow he does not gets prosporous (may be due to his past karmas).
Then in next birth his soul may get entered into prosparous pregnant woman.
Another example :
Person borns in rich family. throuout his lifetime he is doing spiritual practices , but somehow he does not finds a way to liberation.
Then in next birth he may born somewhere near any monk or may get contact in the monk through whome he can find way to liberation.
This is how cycle of birth and death takes place.