You may like to observe this yourself or you can marvel at the sheer idea that it might be true. The idea is this:
When a feeling arises, we attach an arbitrary thought to it from our stock of standard thought patterns. The true nature of the feeling may be completely different to the thought it generates.
(This is similar to the notion that we attach random explanations to events after they have happened - something that has been demonstated many times in scientific experiments.)
So, for example, a feeling of emptiness arises. Our habitual thoughts may latch onto it and start generating "fix it" actions - emptiness is unpleasant, we must avoid it. These very thoughts are a window into the reason why we might be feeling unhappy in the first place ... Or emptiness arises and we allow our initial unease to pass, and we left with nothing but emptiness - which as we abide with becomes something quite pleasant.