The job of your old brain is to constantly scan the environment and answer the questions: “Can I eat it? Can I have sex with it? Will it kill me?” That’s really all the old brain cares about, is food sex and danger. When you think about it, this is important. Without food you’ll die, without sex the species won’t continue, and if you are killed the other two questions don’t matter. So animal brains developed early on to care intensely about these three topics. As animals evolved they developed other capacities (emotions, logical thought), but they retained a part of their brain to always be scanning what is going on for these three critical questions.