Have you ever encounter a moment when you are talking to someone and suddenly the topic in your mind changed because it was triggered by something within the environment and you started talking about that new topic. You have then forgotten about the previous conversation. For an example, you are talking to someone and suddenly a notification sound came from your phone and you looked into it but you are still talking to that someone. The next thing you knew is that the conversation topic has changed. Another example, when you're in a coffee shop talking to someone and suddenly a footballer scored a goal of a football match in the television. The crowd screams goal! And you have forgotten what you have talked about.
All these happened because you have too much in your mind to think. Thinking too much is not bad. In fact, by having too much in your head will never leave you empty. You will always have topics going on in your mind even when the topic ends, a new one will appear. This will make you become more creative in action. But overthinking also has its side effect where you may notice procrastination takes place a lot. This is because you have too much thoughts in your mind that you cannot make up what is right and this will also waste a lot of time.