So every feeling that you carry inside your body, actually depends up on the thought that your brain carries. Positive thoughts are just thoughts that send happy emotions/reactions inside your body. Similarly negative thoughts are also thoughts, but the difference here is just that they send painful emotions/reactions across your body

What does exactly brain do?

It can do one of the following things, based up on what you see/hear/visualize

1) It can carry a existing thought or

2) It can create a completely different thought.

As soon as our brain starts to do one of the above two things, our body tends to react either positively or negatively or can just stay neutral. Positive thoughts gives us happiness, while negative thoughts gives us pain.