It is important to think through one’s courses of actions before acting. This is because there are often many ways of solving a given problem. Some solutions to a problem can be quite obvious. Others require careful deliberation to arrive at. Once a list of possible solutions or courses of action has been arrived at, then one selects the solution that best addresses the given problem. Such a problem-solving process also allows an individual to understand the consequences of his or her actions and to make peace with these consequences. Impulsive acts, on the other hand, often lead to regrets, as the consequences catch the actor off guard. As an example, consider a sudden invitation to attend a party at a friend’s house. I could impulsively decide to attend this party without asking any questions, such as: Who are the other guests? Will I be safe? What drinks or food will be on offer? On the other hand, a careful deliberation of the invitation could help me to understand whether I really would like to attend this party or not. Thinking through this invitation equips me with the knowledge I need in order to make an informed decision—one that I need not regret later.

Being spontaneous is surely fun sometimes, but there is a difference between spontaneity and impulsiveness. When we act without even considering the consequences or potential consequences, or how it may affect or be perceived by others, we're being socially reckless. As thinking human beings, it's much better to make or way through the world as purposefully as possible. We'll do less damage, to ourselves and others, if we do.

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.