I fall into this trap all too often. Deadlines feel too tight to take a moment of frivolous drawing. It seems selfish to indulge in drawing for drawing's sake when the paying projects are nagging at you, but this is just what I did on a few recent outings. I don't regret the choice. The experience of drawing from nature with no intentions other than to capture what was in front of you and make it a permanent expression of your perceptions on paper is a powerful experience. I had forgotten how therapeutic it can be to simply draw for no other reason than to draw. I found myself immersed for a few moments in the shapes and textures of my subject. I found myself really seeing the things I was drawing and solving problems as I worked to get them down in my sketchbook. I was totally invested in the effort and for a moment there was nothing between me and the trees but a thin column of carbon. I became one with my drawing. This is why I draw.
I have learned that what I have not drawn, I have never really seen.
In spite of everything, I shall rise again: I will take up my pencil, which I have forsaken in my great disagreement, and I will go on with my drawing.
-Vincent Van Gogh
There will always be reasons why not to draw. but don't let the ups and downs, the pressures of life and deadlines derail you from your drawing. Draw on!