Well, look at it this way. Have you ever woken up and immediately felt it was going to be a lousy day? Or have you looked out your window first thing in the morning to see that it's raining buckets and thought the same thing? How did your day end up? Did you forget your wallet at home? Did you end up in a fender-bender? Did you get into an argument with a work colleague, a friend or your partner that day? Maybe it wasn't so drastic but be honest, once you decided it wasn't going to be a good day, didn't it turn out to be a crappy day?
How does this happen? It's about energy and Quantum physics. Quantum physics proves that everything is made up of atoms, which are made up of particles called quanta. Quanta is energy vibrating at a certain frequency, which attract other quanta vibrating at the same frequency.
The particles that form everything, including our body (energy powers our body) and mind (thoughts with feeling) energy, are always vibrating so it's energy in motion.
Our thoughts are energy so they vibrate at a certain frequency and attract other thoughts (think "monkey-mind) or other energy that vibrate at the same vibration (getting into an argument with a co-worker). This is why there's the saying, "like attracts like".
It's like a reflecting mirror. You send out your vibrating energetic thought and what is reflected back to you is a like-energy. And depending on the vibration, positive or negative, of the thought you had and sent out, you'll attract the same energy back at you.
But you can choose to direct your thoughts in a certain way. The results of our thought don't manifest right away. They are compounded. It's having one positive or negative thought built upon by another positive or negative thought and the bigger the energy, the more you attract a like-minded energy. But sometimes the results of our thoughts are immediate.
Like attracts like so positive, compounded thoughts through repetition attract positive events and experiences into your life. They are reflected back to you.
You can begin to shift what you reflect by catching the thought. Ask yourself if it's a positive thought. If it isn't, dump it. Let it go. You can also just turn it around. Rather than, "It's going to be a lousy day" (because it's raining outside), think, "It's refreshing to have cleansing rain". It's the feeling behind the thought that will change the vibration. Imagine feeling content as you drive to work knowing the trees are being watered, or, for that matter, that you have clean water to drink because of the rain. You don't have to pull your car over, park, and start singing "Kumbaya". Just find the positive feeling that comes up for you and let that flow out from you.
Practise. The energy of thoughts gathers momentum, and more thoughts that are alike will come to your mind. You can do this to brainstorm, to put yourself in a positive frame of mind, or even to bring calm and optimism into your day.