OVERlife, along with reproduction, is the most basic need we have as humans.
Evolution has provided us with a genius system, unfortunately since the industrial revolution this has come under great strain as our lives have changed 100% over a few hundred years.
The brain stem is responsible for instantiating the oldest functions of our body such as blood pressure, heart rhythm and breathing, for example. In addition, this is the centre where all information from the body passes before it gets higher in the brain.
You can compare it to a spam filter. The filter selects what is of interest to let through to other parts of the brain so that they may or may not enter our consciousness.