What made susan stop believing in Narnia?

Answer 1

Some critics have said that it was simply that Susan reached puberty and discovered sex, and that this was incompatible with Narnia.

This answer is WRONG. Peter also reached puberty, but he did not stop believing in Narnia.

The correct answer is that Susan was distracted by the worldly concerns of adolescence. Instead of trying to integrate her adult life with Narnia, she abandoned Narnia and followed only her worldly interests.

The point Lewis was making is that people sometimes perceive a false alternative. For example, in his own adolescence, he discovered the "grown-up" drink beer. He therefore stopped drinking lemonade because it was "childish". When he was much older, he realised that he was now able to drink both beer and lemonade, and nothing was stopping him having both.

As an adult, Lewis realised he had been a very foolish adolescent. The character Susan represents the part of his adolescent self that he disliked.

I also suspect that he was obsessed with the magical properties of the number seven, so he wanted to reduce the number of earth visitors to Narnia from eight to seven.

Answer 2

Susan stopped believing in Narnia as she grew older and became more focused on materialistic pursuits and societal expectations. She lost touch with the magic and wonder of her childhood adventures, prioritizing the mundane over the fantastical. Additionally, as she became more distant from her siblings and Narnian friends, her belief faded away.