There are rocks, logs, bottles and who knows what else under the surface. It is not very pretty. When the level is low, you can spend days, even months, trying to clean up the bottom of the lake and you won't make a noticeable difference. If you allow the water level to rise, however, the lake will be beautiful again. All the junk is still down there, but it is no longer an issue.
People are the same way.
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.