One's family of birth & economic situation, education & intelligence, bodily characteristics & looks, enjoyment & suffering are predetermined at the time of birth according to his prarabdha karma. And through out his life it keeps changing according to the action he performs. As he sows so shall he reap.
One's attitudes, behavior, character, tendencies and urges are decided by his previous lives samskaras (mental imprints) and his desires. And it keeps evolving under the influence of his surroundings.