Thoughts, much like pollen or germs in the air, are rampant. They are everywhere. They don't originate in our minds, but belong to the collective universe. As Byron Katie is fond of saying, “There are no new, or original, stressful thoughts!”
In the same way germs or allergens affect those with weak immune systems, we, too, are “infected” by negative thoughts, if and when our “mental immunity” is compromised . Only then are we susceptible to the “germs,” i.e. negative thoughts, that surround us.
Our “mental immunity” is weakened by years of believing negative thoughts/beliefs/”stories” about ourselves and others and this leaves us susceptible to the stressful thoughts that run through the mind.
When our “mental immunity” is high, negative thoughts, like germs, may enter the mind but they don't “stick,” they just move on through. Our job is to investigate and adjust any thought that causes us unhappiness so we can build our “mental immunity” against negativity.