If you view history from one perspective, many things can go wrong.
If you don't look at things from another perspective, you only see one outcome or response. Take an example from the perspective or a war. If your side wins, people remember the bravery, the honor, the win. However, the group that was beaten will remember sadness, loss, unhappiness. If you only looked at things from the perspective of the winner, you won't see anything wrong with it. If you just turned around, you would see the losers; you would see that it wasn't a happy victory for everyone.
Another example is from WWII; when the Allies arrived to help, everyone saw it as good and a new hope. For Germany, this was an utter disaster and was very bad. I'm not saying that we should necessarily feel pitiful for Germany, I'm just saying that if we only looked at it from one perspective, we would only see something good instead of something bad. If you only look at the good (or bad) and gloss over the bad (or good), people can only have one true opinion.