Lost Focus During Art Class

I started doodling during art class today, but I wasn't drawing anything from the assignment. Instead, I was sketching my friend Tom's messy hair and his goofy smile. It felt like a waste of time, focusing on something so trivial when we should be practicing our shading techniques or exploring new mediums. Yet there was this strange satisfaction in capturing him that way, making it feel less like classwork and more like personal art. Why do I always get distracted by people instead of the materials?