The 1st arrow is what life shoots at us. These are things that we have to deal with because we are human working with the human condition.
The 2nd arrow is optional and usually filled with Anger, Hate, Frustration or a myriad of other negative emotions.
Why do we shoot ourselves?
It usually comes from our unmindful thinking patterns, better known as habits. Some call it Autopilot. When we give the time needed to investigate them, we can have a better life experience.
Mindfulness allows us to slow things down and lets us see how things really are. With time and practice we can make the decision to respond instead of react to the inner/outer experience.