I have accepted that some fish are going to die no matter how well I care for them, and I do fuss over my fish.
I just think that imported fish are delicate little creatures that have undergone all manner of stress and tribulation to get to us, and that would be enough to weaken them to make easy prey for bacteria or parasites that could cause harm. We've upset the flow of nature by bringing the fish in and taking it out of its natural environment, I think it's a risk inherent with the hobby.
Also it's impossible to know how old many imported fish are, unless you know the supplier - even if they are small they might just be runty rather than young.
Fish bred in captivity should be easier to predict what is bothering them as it's likely you'd know a bit more of the history, but again, unless you're a biologist and a vet, you may never know what killed the fish.
I think it's Jaxxnz whose signature is 'fish come and go' and I think that's pretty true.