A lot of students want to use part of their college application to explain or justify something they're not proud of, like earning a low grade, quitting an activity or choosing not to enroll in a difficult class. Some scenarios call for explanation, but most don't.
Four of the following five explanations would likely be seen by admissions officers as just excuses.
A) I had a personality conflict with my chemistry teacher my junior year and because of the grade I received, I was not allowed to take AP physics.
B) Unfortunately, I received a D in US History because of my intensive commitment to my activities.
C) I choose not to continue playing soccer after my sophomore year because of the politics on the team.
D) I was suspended from school my junior year because a teacher falsely accused me of cheating.
E) I couldn't take AP calculus because it was only offered at the same time as AP English. English is my favorite subject, so it won the academic face-off with calculus.
E is a good reason and worth sharing. The others are just excuses.
The message here is not that you're a bad person if you make a mistake. The message is that if you use your college application to try to explain away a perceived failure or shortcoming, you run the risk of just shining a light on the very thing you're trying to explain away. It's like going on a first date with someone and in the first five minutes saying, "Before this goes any further, let me take my shoes off and explain why my toenails are so grotesque." You would have been better off holding on to that explanation.
There's one exception to this rule–when a college's essay question asks you to share a weakness, failure or mistake. When they ask, that's an open invitation to share. But you're almost always better off owning your choices than you are making excuses or blaming other people.