When you persist in the practice of self awareness/observing your thoughts/meditation - all of these words are fancy synonyms, you realize that
some feelings apparently coming
from nowhere are, in fact, coming from super rapid thoughts that went unnoticed.
But now you notice them.
And you notice one more thing:
The more quickly a thought appears, triggers an emotional response and then disappears, the more it is so because this
thought is "automatic".
You continue with the process and realize now that automatic thoughts are just "trained" thoughts, that your super-heavenly-designed brain
has optimized for you.
Like a shortcut in your Google
search bar :
"You engage with this thought everyday ?"
"Let me facilitate that for you and make it super easy:
YOU WON'T EVEN HAVE THE SENSATION OF THINKING!"
Ha ha! What a nice idea, Mr Brain... you can't even fathom how many of your humans you're losing like that. 
What, in your life, is made of automatic thoughts?
Well, it's quite the big stuff:
1- Your mood: your mood is nothing else than an assesment, every morning when you wake up (and anytime of the day), of your life situation.
If you take the habit of not liking it - i.e. noticing how it may differ from the life you want, how you wish you'd be somewhere else - instead of what it already has
in common with it, you create a default wake-up mood made of sadness, frustration and not wanting to get out of bed.
Personally, that's one of the first experiment I did on myself. I changed my mood upon waking at 23.
2- Your identity: Since your mood is indexed on how you perceive yourself in your own life, your default mood everyday dictates your
sense of identity.
Who you think you are, what you think you deserve, what you feel legitimate to claim and go after.
Why is it important?
Because here's the thing: it corresponds exactly to what you get.
Your current life situation, in every aspect, is the mirror of your sense of identity.
It is what you're settling for.
What you think you're worth.
Big changes never happen unless you reconnect to your desired identity and redefine a new normal.
Big changes never happen unless you work on your thoughts.
 Well, your brain is doing you a service here : you ask him to think a thought everyday. You actively think this thought everyday.
Like with every other repeated action, the brain optimizes and does it now without you even noticing: learning how to drive,
checking your phone..etc. You basically taught him.
And I like to stress that point because it gives you your power back : you really can teach your brain new automatic thoughts.
Become aware of the existing ones when they appear and then actively think the new ones.
It really is about repeating. Until your "Google search bar shortcuts" change.