The way our minds usually function involves a continual push-pull between past, present and future, only spending fleeting moments actually being here. When we're not occupied with thoughts like "what am I going to do when I finish what I'm doing right now?", we're occupied with thoughts like "why isn't what I'm doing right now as good as that thing I was doing before?". We're rarely here now, which may sound clichéd but is easy enough to verify as accurate in our own experience; we're either fretting over what might happen in some not-occurring-right-now future, or worrying about what's already happened in, or may occur due to, some not-occurring-right-now event in the past.