This has many threads.The first thought of course is that thinking is about control. We are not allowing things to happen to us, not waiting to respond naturally. Instead we try to think in a way that we can control the entire process. This controlling nature is what leads to a thinking, overthinking mind.
So, what's wrong with thinking?
The fact that we are thinking ahead means that we are trying to control And control is a fear based reaction (mostly). And that takes away from our nature, our natural way of responding to the world, to life.
Which means ...
That thinking comes in the way of being ourselves. The more we control ourselves (thanks to our thinking) the less we are being ourselves and the more boring we are as people. It is only those who are truly free from thinking, from controlling, who can be themselves as they are - and they are most interesting.
So then, stop thinking, stop controlling and you will be more you. You will experience life so much more deeper and more colorfully.
Don't think, do. Like Maverick says. Or the Zen masters say.