well I think the case of Oba talk by itself and it's useless to talk about it.
The other two case (jennsen and Drefan) are very different each other.
jennsen grew up with the love of her mother and I think it helped her to not give in to the voice and of course her mother teached her the right things of the life like respect,love, justice and so on...
Drefan instead grew up without his mother's love,and I think it is the worst thing that could happen to a child, or at least he understood this (I think his mother loved him a lot and that's why she left him with the priest: to offer him a better life), and in the difficult periods of his life (like adolescence) without anybody (familiars) taking care of him he found a friend in the voice and gave in to it only to feel better than before.
" I'm the one of the prophecies. I AM Fuer Grissa OSt Drauka. I'm the true Seeker of Truth because I'm the only one who could make the blade become white. I'm Lord Rahl and I'm searching my Mother Confessor......."