People should think before they act for a number of reasons.
First of all, not thinking before you act can get you into legal trouble. You can, for example, be sued for negligence if you do not exercise reasonable caution when doing things. So, for example, let's say you throw a large rock out of the window of your third floor apartment. You did it without thinking or meaning to hurt anyone, but it hit someone and hurt them badly. You could be sued for negligence or charged with the crime of reckless endangerment.
Second, even if you do not end up in legal trouble, you can ruin relationships by acting without thinking. For example, what if you believe something mean about a friend and say it to them without thinking? You could easily destroy your relationship with them simply because you did not think before you spoke.
So, failing to think before you act can harm you in many ways.