Positive thoughts build up that which they are about while negative thoughts destroy that which they are about. When you are thinking positively about what you want, you are building it up, but when you are thinking negatively about it, you are destroying that which you have built.
The more positive are the thoughts you think about that which you want, the larger, stronger, and more robust does your vision and its corresponding manifestation become, but the more negative are the thoughts you think about that which you want, the more decrepit, destroyed, and dismantled does your vision and its corresponding manifestation become. This occurs, because the way you feel is your point of attraction and your point of attraction always matches the vibrational average of the frequencies that are given your attention.
Due to this phenomenon, when you think positively about that which you want, you attract experiences that vibrationally match those thoughts, experiences in which your vision and its corresponding manifestation are robust and in good health, but when you think negatively about that which you want, you attract experiences in which your vision and its corresponding manifestation have been dismantled and destroyed. For this reason, you should do your best to make all of the thoughts you think about that which you want as positive as possible to build it up and make it as strong and as robust as it can possibly be so that your life experiences will then match those thoughts as a result.