Thinking about the future, is not the problem here, it’s excessive thinking and worrying about it that can be derogatory. Miss Maudie was telling Scout that, while preparing for the future was a good thing, worrying never gets anything done. If you are constantly worrying about what might happen next then you never actually get anything done to prepare and it scares you, turning it into a counter-productive waste of time. The future is so we can wonder and hope for the best, but have time to prepare for the worst; the present is our actual preparation time, when we gather our lessons and ready ourselves; and the past is for our reflection, so we may learn lessons from other’s experiences and not have to learn them the hard way. While the present may overlap the other two it is important to remember that it is still a separate time and needs to be treated as such.
I hope that was sorta-deep, or at least made you think!