You found this brush that works really well.
You use it for fine detail. It holds up nicely. Never lets you down.
Then one day you see a weird flake fall. You wonder where it came from. You keep painting. Another flake falls. You realize the finish on the handle is flaking off. Oh well, that doesn't matter.
After a while, there is no more finish on the handle, just bare wood. You've become used to the rough feel of it, it feels nice, it feels comfortable.
One day, you go to use the brush and things are laying down real nice, except then you notice a strange line that keeps appearing. You work it in, wondering where it came from. Soon you notice that there are a few hairs on the brush sticking out at an odd angle. Ah, that's where that line came from. No problem, you say as you work a little on the brush trying to get the hairs to all line up. Nope, they refuse to cooperate, they continue to stick out.
You go ahead and use the brush anyway, trying to keep those wayward hairs away from the canvas. Fortunately, they are all together on one side, so it isn't too hard to do. Soon they are sticking out further and further. It becomes more of hassle then it's worth.
Yesterday, I bought a new brush that will hopefully take it's place. Haven't tried it yet, but I'm trying to keep a positive outlook.