The mind can only carry one thought at a time. Whether that thought you are thinking of is negative or positive, you choose it. So whenever you come across a negative thought, you will substitute that thought with something positive in order for you to reinforce and reiterate the positive thought to wash the negative thought away. If you choose the negative thought, you will more than likely carry this as a burden which eventually cause a break down in your happiness and well being. Would you rather have a garden full of flowers or weeds? The weeds represent the negative thoughts and the flowers represent your positive thoughts. The weeds are destructive to the garden, just as a negative thought is destructive to the mind and the flowers are beautiful for the garden, just as a positive thought is beautiful for the mind. It is always encouraged to substitute your negative thoughts with your positive thoughts, as this will enable you to be happy and lead you to a more successful lifestyle.