Considering one's own personal outlook in a situation where there are no others around - such as is the case with the moral of The Fox and the Grapes - is not considering behavior towards others. — creativesoul
I'd like to resolve this, but it's fine if you'd like to move on.
I've identified two morals in the fable, which are insufficiencies in self-reflection and a good work ethic. The traditional interpretation is that it's just about a good work ethic, I understand. I believe the lack of self-reflection is the more significant moral, personally.
Regarding the traditional moral, there is nothing wrong with seeking low-lying fruit. In fact, it may be more ethical to stay within your natural niche, rather than working hard to expand it, because you may be disrupting the natural balance. Other species, and perhaps eventually yourself, could pay a price for disrupting the natural order too aggressively.
The implied work ethic that we should work hard to exploit natural resources beyond our natural reach is obviously cultural in origin, and actually rather disturbing. And it's meant to program children!
That the fox considers itself a failure means that it has adopted the work ethic and has failed to live up to it. So, not its own outlook and not failing itself but the expectations of those who imparted the ethic. The failure only matters in relation to others, because as I've previously mentioned, her energy might be better spent moving on and seeking low-lying fruit elsewhere. Her lack of industriousness is of concern to society and not necessarily to herself.
Incidentally, there's a compelling argument that hunter-gatherers had to be forced into agrarian life because it sucked compared to a hunter-gatherer lifestyle, at least for many hundreds of years.
We might also consider the meaning of "behavior towards others." Clearly, we can express behavior towards others that are not in the vicinity of ourselves, and our successes and failures effect others. It might be claimed that the fox's failure doesn't effect anyone else, but if that's the case, why does the fox need to contrive the fiction that the grapes are sour? If it's to console himself, why does he need consolation? We don't console ourselves with comforting fictions every time we feel frustration and failure. The fox is attempting to save face, to put it colloquially, and this relates to others. If there are no other actual witnesses it could just be an ego driven habit. If the fox were truly a solitary animal it would have no such habit, or it would eventually fade away if the fox became truly solitary.
The fox's moral code includes a particular work ethic that it betrayed. It doesn't matter if anyone else witnessed it or was directly effected. It's like breaking a promise that you've made to the society that you belong to. If it's a good promise to make then you've let down your community in whatever actual benefit the promise is supposed to encourage. You've also diminished the value of the promise itself by your unfaithfulness. The community bond is weakened.