I don't change my main bathroom towel as much as I mean to. There's a very simple reason: almost every time I'm about to change it, I think to myself, "I'll just use it for one last little thing before I change it." Then, once I use it for that one little thing, I forget to change it.
This makes sense to me on some level, at least when I'm not really paying much attention to what I'm doing, having done all of these "bathroom things" thousands of times before. After brushing my teeth, there will likely be some sort of toothpaste residue on my mouth that I wipe onto the towel. After showering, the towel will be wet and covered in the little bits of dead cells and whatnot that comes off naturally when you dry yourself. I usually think of changing the towel when I first get into the bathroom, and decide to do so as soon as I'm done doing whatever I went in there to do. But it never happens, so I only end up changing my towel once or maybe twice a week.