Because people never stop and think about the consequences of their actions before doing them. If people took the time to think about things logically they would hardly ever make mistakes and the mistakes they did make would be so small they would be easy to fix.
You must realize that all of us are imperfect, we are going to make mistakes. What's most important is that you learn from these mistakes. They are life's teaching lessons and we all will go thru them. So when u make that mistake, learn from it so that you won't continue to make the same mistake over and over again! Make sure you love life and live your life even thru the mistakes! Take care