Because most of us are lazy at one level. You may be an exception but at least I am lazy. So, if I don't have any goals, my mind wanders all the time. I would do many things my mind is attracted to without any long term vision. Then, probably I don't get what I really want. Soon, the time will be up. The end of my life. The story ends.
If you haven't recognized what you really want, what really matters to you, then it is the time to start thinking about it. Life is one time thing. There is no second chance. Be thankful for being alive now and be clear on what you really want in life.