As per the Hindu Vedic teachings, a soul will not die; but only the body perishes. But the soul after leaving the body carries its virtue and sins that has been earned during its life time. The Soul can not take a rebirth immediately. It has to live in Pitra Loga or Pithru Loga (Anscestor's World) after its departure from the Earth.
The Pitra Loga or Pithru Loga's one day equals our Earth's one year. Thus doing Pitra Tharpanam or Pithru Tharpanam once in a year means feeding and remembering your departed anscestors on daily basis. If a Soul fails to get this service done by his Sons/Daughters, then it has to wait for 100's of years for a rebirth; still such time arrives, the Soul waits for its turn in Pitra Loga or Pithru Loga.
Angered by the delay, the Souls curse their own siblings. This brings the worst form of Pitra Dosha or Pithru Dosham. This leads to cascading effects on Marriage, Child birth, Harmonius living and physical and mental illness on the entire family.
If a person performs Pitra Tharpanam or Pithru Tharpanam on the stipulated days, the Soul would be able to prepare for another rebirth; so that it will have a chance to earn good virtues and offload all sins. Thus the Soul becomes very happy and bless their siblings.
Thus a Soul would be able to arrange for its rebirth; rebirth would be calculated on the basis of its virtue. A Soul with good virtue will make a rebirth in a family that can deliver good things in the lifetime. Similarly a Soul with lot of Sins to its credit will end up in a family, where there are lot of sorrows and sadness that surrounds the family.