What doctors don't readily tell you is that your thoughts create your emotions. This means literally that the things that you think cause a chemical reaction in your brain that release hormones/ chemicals that cause you to feel emotions. To clarify, you don't just become sad. You had to think about something first that caused your body to release a chemical that created the emotion of sadness! This is not philosophy, this is science my friends!
Thoughts also have frequency. Thoughts, emotions, & frequency are measurable in terms of Hertz. Did you you that you are an electro-magnetic being? Your frequency attracts or repels frequencies that are on the same wavelength. When you "raise" your thoughts by thinking positively, your frequency also raises. This is why they say "like attracts like" and why The Law of Attraction is a best selling book; because it's true and it's physics!