The face of a woman, a child. Hands. The expression of a mouth. These are relatively hard to draw because we know what they are supposed to look like because we see them so often. We have to get the details right up to fractions of millimeters, or else it will look wrong.
Trees and clouds are organic. They can look like anything, and you don’t have to be as precise for them to look good.
Dragons, too, don’t have to be drawn as accurately. Who knows what dragons look like? You can vary the details greatly, and it will still look good.
I wrote in another article that to form a habit, it is essential that you are happy with the outcome. Drawing things that are easy to get right and look good helps greatly.