It’s that they don’t know how to stop overthinking. They overthink every little problem until it becomes bigger and scarier than it actually is. They overthink positive things until they don’t look so positive anymore (and as the anxiety starts to build).
Overthinking is destructive and mentally draining. It can make you feel like you’re stuck in one place, and if you don’t act, it can greatly impact on your day-to-day life. It can quickly put your health and total well-being at risk. Rumination makes you more susceptible to depression and anxiety.