When I saw this question, my mind spun off into multiple directions, trying to think of all the things that “others” see as success, that I don’t. When I tried to summarize them all, it came down to this:
“Success is internal to you. You succeed when you feel like you have, and nothing else.”
Nobody else’s definition of success applies to you (if you ignore the paradox this creates with my statement above). If you want to appear successful to others, sure, go ahead and find their definition and try to conform to it. But you’re truly successful only when you feel it - even if nobody else can tell!
As for why, I can put it down to experience: there have been plenty of times I have felt successful, when others did not see or even realize it, and there have been a comparable number of times when others have expressed delight in their own success, but where I failed to see it. In fact, I have reached a point where I do not think about whether or not someone else has been successful, but instead, I watch out for cues that suggest they feel successful, and compliment them or encourage them to achieve further success without suggesting terms for their future success.