I created my own X11 window manager [1] at the start of this year in around 800 lines of C.
I had been using dwm (4000 lines of C) for many years and wished to write my own for a long time, but what made me take the leap was really steveWM [2] and TinyWM [3] which are both super small.
Was a lot easier to understand for me than, say, dwm.