Because you’re basically still a caveperson.
One of the more amusing things you will notice in meditation is your brain’s ability to escalate wildly. Your mind has an amazing capacity create much bigger problems. Say, for example, you feel a slight twinge in your back when you’re focused on your breath in meditation. Left to its own devices, your brain will be off and running like this:
Fuck! My old sports injury is back! It took years to recover the last time. My physio is really expensive too, he put up his prices last year. Fuck! Fuck! Fuck! I’m going on holiday next week and I don’t want to have to put up with serious back pain when I’m in Greece. That will be shite.
All this will take a fraction a second.
Dealing with this in meditation is simple. Notice your mind has wandered off, and gently shift your focus back. But it’s worth noticing the exaggeration process. You’ve gone from a tiny fleeting sensation to an expensive, debilitating condition that’s going to ruin your life. This is what brains do because…
Mentally – you are Stone Age
Your environment has changed enormously in the past few thousand years, but your mind is still wired to think everything is out to kill you. It was a good survival instinct at the time but Captain Caveman still rules your mind today. He’s always scanning the horizon for things that might go terribly, terribly wrong.
Given enough practice, you will get good as spotting when this happens. If you walk into work and your boss is unusually formal with you, you may notice that your brain is running away with itself thinking up reasons to explain why that would be. You can spot the daft theories about your boss not being happy with your finance report, or that HR has somehow caught you binge watching Great British Bake Off when you were working from home.
Catching your mind doing these bonkers somersaults and cheerfully ignoring them is one of the great skills to learn from meditation.