The complete quotation from Carl Jung is "What you resist, persists and grows." The reasoning behind that quotation is that if you resist something, you are giving it your energy, although it may be negative energy, it is energy and is available to be transformed and used for growth.
A perfect example in our modern American psyche is our obsession with resisting Trump and the radical Republican right wing politically. As you may have noticed, the more resistance they encounter the more they seem to grow in strength.
Trump's ability to cause sensations lured the press to follow him in his presidential bid in 2016 and eventually put him in the White House and put the Republicans politically in charge of most of the nation. Much of the country resisted the Trump candidacy and the spectacle only grew.
Since that time, the Democratic Party and many others have been obsessed with resisting what Trump represented while watching his appeal only seem to grow.
The mistake the Democratic Party and many others made and continue to make is to focus on resisting Trump and the Republicans instead of putting all that energy into finding positive alternatives and solutions into which they could channel all that power and energy.
A perfect example of the truth that whatever you resist, persists and grows.