For me it's usually like this:
I sit down, start watching the fish and chuckling at their antics. Then I notice something, usually a dying leaf, or damaged Vallisneria after my bristlenose's nightly adventures. I try to ignore it, telling myself it's just one leaf, it's barely noticeable, I don't have to take it out right away. But my eyes keep getting drawn to it more and more insistently, until I absolutely have to grab my tools and remove the offending leaf. At which point I always notice another. And another. And another. And doesn't that plant look like it needs pruning? The glass looks a bit dirty, doesn't it? I think I'll clean the powerheads too, while I'm at it. Oh look, another dead leaf. There, pruned the plant, but do I even like the plant where it's growing? Maybe I should uproot and move it. Oops, brought out plenty of detritus from the substrate, time for a water change... And so on.