Parshas Toldas

Yitzchok and Rivka prayed to be blessed with a child. Both of their tefilos were the same, yet the posuk tells us that Hashem listened to the tefilos of Yitzchok over Rivka. Rashi explains that the tefilos of a tzaddik who is the son of a tzaddik are much more powerful than that of a tzaddik who is the son of a rasha, hence Yitzchok's tefilos were preferred over Rivka's.
The obvious question is: If both were great tzadikim why does it make a difference what their parents were like? Or better yet, Rivka's tefilos should have been more dear to Hashem being that she had to work on herself as opposed to Yitzhock whose greatness came naturally?
I heard the following answer from R' Yakov Salomon (b'shem Rav Dessler Vol 2 pg 205) at the bris of his grandchild, Yeudah Garfinkel- (mazel tov!). For the son of a wicked person- someone who wasn't brought up with the teachings of Hashem- to find hashem on his own is very noteworthy. Though once he finds Hashem the path he takes in serving him comes naturally- for he has nothing to emulate. On the other hand the son of a tzadik knows from the day he was born that there is a creator of the world, yet he still faces great challenges. His challenge is to find his own path in avodas Hashem- not just to serve Hashem the way his father did- rather he has to serve Hashem because that is what he himself feels and knows is right.
We start shemona esrai with 'Elokai Avraham Elokai Yitzchok v'Elokai Yakov'. Why dont we just say 'Elokai Avraham Yitzchok v'Yakov? Why do we preface each of the Avos with Elokai? Avraham found Hashem through the middah of chessed. He realized that Hashem is the fountain of chessed in the world and therefore sought to emulate Him by living a life entirely made up of doing chessed. Yitzchok could've just served hashem the same way, the way he was brought up by his parents- chessed. Yet he went out and found Hashem on his own, through his own middah of gevurah. So too, Yakov found Hashem on his own through the middah of emes. We say 'Elokai' before each Av for each one found Hashem on his own, with his own strenthgh and own personality. It is this that gave us three Avos, each one with his own distinct avodah. All three together form a unit that was able to father Klal Yisroel.
With this in mind we can now understand why Hashem prefers the Tefilos of the Tzaddik the son of a Tzaddik (Yitzchok) over the Tefilos of a Tzaddik the son of a rasha (Rivka).

Thank you Menachem Schwartz from yerushlaim for his input.