When their level of well-being drops, their world starts to look quite threatening. They are likely to do, say or think any number of ugly things. As with the lake, this ugliness will always exist at low levels - you can't clean it up. It may still be down there, but it is not an issue. It is important to understand that a high state of mind is the natural condition and not the exception.
Little babies are high most of the time.
Now babies certainly have unpleasant moments but their crying is typically their way to communicate physical, rather than psychological, distress. It is only when babies get old enough to start thinking that their moods start to appear. In fact, it is simply the weight of thoughts that drags down a person's level of well-being. If so, you were closer to the truth than you may have realized. This is obviously a subject that deserves to be explored in more detail but for now just understand that it is the quantity of thoughts that brings down a person's level of thinking and that as the level of thinking gets lower, the quality of a person's thoughts become more negative.