In psychology there is a concept known as the fundamental attribution error wherein a person has a tendency to overestimate the internal conditions playing a role in others' behavior and overestimate the external factors affecting her/his own behavior. For example, if a student shows up late to class you might assume it's a part of their personality to be lax with time or careless. But if you show up late to class you "know" it's because the bus ran late or all the stoplights were red. External circumstances probably play more of a role in other's behaviors than we are aware of and likewise our own behaviors might be a little more connected to our personality/character.

This occurrence has grounds in science. Humans like to fill in the blanks and make sense out of things, and if something is unknown the brain automatically develops an answer, be it true or untrue. So when we see someone lashing out on the perfume salesman, brought to tears by a cheesy movie, or standing on a table taking shots, we assume things about their personality. But can you think of circumstances that those people might have been in, or past experience they might have had that caused their actions? You don't know them so you can't see the full life they have behind them thus based off of your small amount of information your brain induces this personality attribution simply because it is the most logical answer.

And this is why we are so easy on ourselves; we have a full warehouse of our past experiences. emotions, traumas, learned schemas, etc. that explain our actions independent of our character. If you show up late to class it's because you couldn't get out in time because you forgot you had to feed your roommate's cat and you've never had a cat before so of course you're going to forget you have this extra task and it's not your fault. Life is done unto you and you shall not be held accountable. This is an extreme example meant to demonstrate that we can justify our actions thoroughly. And this concept is merely a concept; knowing we might have this tendency can heighten our awareness and help us to cut others some slack and tighten up on ourselves. Isn't meta-thinking fun?

Source: Levy, A, David, 2nd ed., 2010, Tools of critical thinking: The fundamental attribution error: understanding the impact of external influences, Long Grove, IL, Waveland Press, INC.