Question about 50/50 negative/positive emotions concept

Why does Brooke believe that we experience negative emotions 50% of the time and positive emotions the other 50%?

Since our thoughts determine how we feel and we have the power to choose our thoughts, don’t we have the power to choose how often we feel positive and how often we feel negative based on the thoughts we choose – whether that’s 50/50, 70/30, 80/20, etc?