The way we perceive others and how we attribute their actions hinges on a variety of variables, but it can be heavily influenced by whether we are the actor or the observer in a situation.
When it comes to our own actions, we are often far too likely to attribute things to external influences. You might complain that you botched an important meeting because you had jet lag or that you failed an exam because the teacher posed too many trick questions.
When it comes to explaining other people’s actions, however, we are far more likely to attribute their behaviors to internal causes. A colleague screwed up an important presentation because he’s lazy and incompetent (not because he also had jet lag) and a fellow student bombed a test because she lacks diligence and intelligence (and not because she took the same test as you with all those trick questions).