Because his brushstroke is unique. I'm no art buff, but I know what I like when I'm looking at it.
My favourite of his all must be 'Irises'. There's a mystery to the single white one, amongst the other 'normals'. The overall colour blend is nice - blue/indigo and green, the accompaniment of oranges on the top left corner, dashes of yellow, and earth on the bottom left corner.
On the subject of art, my kind of art is more towards geometric and definite. No wonder why among my past ambitions was to be an architect. Well, I guess a mechanical engineer suits me too. Once in a while I get to draw, those geometric and definite lines with Autocad. If I really have the time, I'd go old school: pen, paper, ruler, stencils and geometric stationeries.
I remember during art classes through my school years , I'm more keen towards letterings/typesets, logos, posters and such. I guess that's why I'm into XS - because of the definiteness, while offering a work of art when it's finished.
Some time ago I attempted to convert Irises into XS sheet so that I can make it a major project. However, it turned out to be quite massive and requires many, many different colours. Well, having the painting as my office PC desktop wallpaper is okay too.