Don't start a PhD if you think it will make you more confident as it is very likely to do the opposite. And you might end up deciding not going on in the same career path anymore.
Don't start a PhD if you think you need a PhD before you can contribute to the welfare of mankind. Or if you think life will be meaningless to the point of warranting a suicide if you quit your PhD one day. Don't start a PhD if you think you need a PhD to have a happy and meaningful life.
Don't start a PhD if you have a hypersensitive personality or suffer from depression. Your PhD might very well be a gateway into the dark side of you.