A little crass perhaps, but there’s a deeper point. Montaigne was hinting at a fundamental shortcoming in how we think — existential asymmetry. It affects all aspects of life, but we’re mostly blind to its effects.
Your “data” on yourself is complete (relatively) and easily accessible. In contrast, our understanding of others is always incomplete and mostly inaccessible. We are time-bound, closed systems aka existential.
This means we have intimate knowledge of our own failures and shortcomings, but not of others. In contrast, other’s successes and strengths are readily accessible and visible. However, their failures and struggles are mostly obscured.
But wait, there’s more! Consider these two mechanisms that make existential symmetry particularly potent in distorting our view:
Discounting and hedonic adaptation — We underplay our successes as flukes, or don’t give ourselves enough credit. Even when acknowledged, over time we go blind to their significance. Once we’ve reached a goal, we readjust to a new target, putting us back in deficit mode.