There is something nice about messing around with felt tips and a biro with a firmly chewed end. It feels like being a child again, frees you up and stops you worrying about the correct way to outline a tern's knee.
I believe that anyone can draw but that many of us have a tendency to fret over things being not quite right. It's your sketchbook and you are supposed to fill it with the imperfect, with smudges and scribbles and that scrap of something or other you fancied sticking in one rainy Tuesday afternoon, just because it cheered you up. Sketchbooks are wonderful things, better records of thought and skill than most diaries, and, I venture, much less painful to look back on.