You mind functions like a bedroom and to me the most important room in a house is where you sleep. Statements like 'As you make your bed, so you sleep on it' tells us the importance of keeping our sleeping area neat and clean. The mind is where the issues of life arises e.g. thoughts, love, peace, desires, self-control, motivation, faith, hope, etc. Please note that the most important things of life are those we do not see, and are connected to the mind.
Every action has its origin from the mind, and most often we are warned to take care of our thoughts because they become actions. Just like how neat or dirty your bedroom can affect your sleep pattern, whatever we allow to occupy our minds can either build us up or pull us down. 'This explains the reason behinds nightmares after engaging the mind in a scary movie'. Positive thoughts are refreshing and helps nurture the mind while Negative thoughts trails and hunts the originator. Having negative thoughts is similar to allowing a wild animal into your bedroom, which eventually won't let you sleep and may end up destroying you or your properties. Daily and most often we have a huge task in checking and filtering what we allow as thoughts.
Thoughts do build up, and whenever we continue in a line of thought we are simply adding energy to it. Just like heavy or dark clouds builds up for a rain, so do continuous thoughts build up. The duty also lies on us to diffuse such thoughts if they are negative.
Some people snap out or rebuke immediately each time the recognize a pattern of negative thoughts. In short, the first step to becoming a Child of light is by constantly check your thoughts and make sure they align with that of your Heavenly Father. We are what we think, and the mind and thoughts are powerful tools for transformation. Have a lovely day and remain blessed.