When G-d rejected the offering brought by Cain, Cain was angry and crestfallen. But Cain remained dejected, and that ultimately brought him to a far greater sin. Rabbi Yeruchem Levovitz explained that remaining dejected over a sin, rather than rising up immediately to correct it, is a result of the evil inclination's work. A sin knocks a person down, but it's the evil inclination that holds him there, overwhelming him with guilt that prevents him from rising and repairing the damage. Staying down, says Rabbi Levovitz, is worse than falling in the first place.
Guilt is the fuel of teshuvah .
The child was thrilled. He hid and waited, while his grandfather sat down at his desk to learn. After waiting quite awhile, Yechiel emerged from his hiding place weeping. Grandfather, you forgot all about me. After eating the fruit and hiding from G-d, Adam would have been too ashamed to come out.