Nature calms people down. It calms you down. 

Your earliest memories of oil painting were of seeing a tree on the edge of a lake in your mind, and the image calming you down, the image drawing you in by its absence of anxiety, its calming effect. 

When other kids were drawing and painting dragons, giant robots, Batman, lions, you were drawn to this image (and you also created some of the imagery the other kids created). 

Must the nature images be calming?

Your current one is. The few you’re willing to show are calming, or calming-ish. They can also be dramatic.

You need open spaces and a clean bottom edge. Those needs come from someplace, unlikely it’s just you. These aspects of visual language doing something for you. They’re worth pursuing.