Brian couldn't get the bow and arrow to work, so that meant that he couldn't get lots of food. Brain was giving up on the fact that he couldn't get the bow to pull back without it breaking or something like that.
Brian was getting really mad/sad at himself because he didn't get the search plane's attention. Brain was at the point of giving up, so he decided to cut himself with the hatchet.
Brian was outside and saw his mom with a random blonde haired man. That is when Brian had "The Secret."