Mostly because of a natural bias we all possess that leads us to want to believe we we fail its a matter of circumstance but when others fail its their fault. We can be consious about it and try not to let it affect us, but in some level it's difficult not to believe this.
Also, because we value different things. Someone with a high value on rational intelligence may thing they are better than people who have a lower rational intelligence than them. Alternatively someone with a high value on relgious devotion may think they are better than someone who doesn't have said thing, or has it but for the "wrong" religion. And these are just two examples of many.
Couple that with life experience that suggest that it's common for people to fall short of the standards you hold yourself to, and it's easy to judge them. After all, you believe these standards are imporant, and you met them, and they don't.
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Also, because we value different things. Someone with a high value on rational intelligence may thing they are better than people who have a lower rational intelligence than them. Alternatively someone with a high value on relgious devotion may think they are better than someone who doesn't have said thing, or has it but for the "wrong" religion. And these are just two examples of many.
Couple that with life experience that suggest that it's common for people to fall short of the standards you hold yourself to, and it's easy to judge them. After all, you believe these standards are imporant, and you met them, and they don't.