I don't often lose fish anymore (except I do have trouble with Cichlids) but when I started out in sw... I killed many.
Frankly, I think the concept of "never losing a fish" is more a function of luck than skill. If you've never lost a fish, you're lucky. Skillful yes, but there is no level of skill that can guarantee this, unless you are wise enough not to get into this hobby in the first place.
Things happen. I had a puffer once that broke a tooth on something in the tank. He never ate right after that and ended up starving. I don't consider that my fault or a lack of skill. Other members here mentioned power outages, etc.
Poop occurs.