3.
Fear
Not knowing how things are going to turn out can cause stress and anxiety which can manifest as anger. Being afraid of death, losing loved ones, illness, car accident, job loss, etc. can be devastating your on your life you let it.
Lots of times something happened in the past that makes you fear that it might happen again. It's perfectly reasonable to feel that way, but if you live in a state of worry and panic because things could go wrong it will make things in your life a lot more complicated and stressful than they need to be.
Realize that some things are out of our control and that bad things will happen to everyone. It sucks, but it's life and hopefully you will have more good days than bad ones. If you live each day in fear that something is going to go wrong then you won't be able to appreciate all that goes right. Next time you're upset, take a moment and decide if your actions are coming from fear or something good.