What does the following quote by Nagato mean?

Question

Even innocent foolish children will grow up in the face of pain, until their thoughts and beliefs are same as their doubts

How can one's thoughts and beliefs be same as their doubts? I can't seem to understand this sentence. One of the biggest reasons I watch Naruto is because of its extraordinary philosophical background.

My reasoning on this quote: Children who grew up in war prone areas always doubt if they deserve to live on this planet or some form of source that ends the suffering. So, here their doubts are, is there really a chance for me to live without pain? An intuitive assertion being that, such a form of life will be impossible and cannot exist and the only way to live is to try and survive against all odds and doing whatever it takes. This enters their belief system and intern imparts their thoughts making them accept the fact that life is not fair for them. Is this interpretation correct or is there a different one?

Answer 1

This is just another manifestation of Nagato's bitterness towards the world. He faced pain, repeatedly and more so than he deserved. That experience planted the seed of doubt, doubt that pain could be avoided unless power was attained. And that doubt entrenched itself more firmly in his mind until it stopped being a doubt. It morphed into a surety. It became belief. That innocence where pain was an unfortunate hardship was stripped away. Pain became an unyielding reality, unavoidable unless enough power could be consolidated to end all war.

This is what Nagato meant. That what began as a doubt in an optimistic view of the world reaffirms itself so forcefully and repeatedly stops being a mere doubt and affirms itself as belief, as reality, as truth. That there is an inevitability in losing an idealistic innocence and facing a harsh reality, changing from an optimistic attitude to one of cruel realism / pessimism.

And herein lies the fascinating contrast between Nagato and Naruto. Both lead a life filled with pain, disappointment, sadness and grief. One embraced positivity, optimism and hope while the other succumbed to negativity, pessimism and despair. One chose to be built up by his trials. The other was destroyed by them. The evident lesson here is one of choice, of choosing a positive attitude and seeing the divergent paths to which a choice in ideology can lead.