Rick and Lori have a very good relationship, being husband and wife. Even though Lori had sex with Shane, Rick thinks nothing of it; he figured she had no idea he was alive, so it didn't matter.
Rick and Carl have a very good relationship, being father and son. Rick was believed to be dead by Carl, but when Rick showed up, Carl was very happy.
Rick and Shane have very good relationship; they are best friends. Even though Shane had sex with Lori, Rick thinks nothing of it; he figured since they had no idea he was alive, it didn't matter.