A few thoughts:
I think Shane wanted to be Rick, he was trying to replace him in Lori and Carl's life, but deep down he knew Rick was stronger than him.
Rick's marriage was shaky before the infection, so he didn't confront Shane until he pushed the his fathering ability. This entire story is about Carl, him growing up in this world, that is why Sophia and Dale died, Rick makes most of his decisions based on how it will effect Carl.
Next week should be awesome