Theory:

      Then he opened his wings and flew right away into the blue. But the little Princess burst into tears, for it is very difficult to put the happiness of someone you love before your own, and with her little bird far out of sight she felt, all of a sudden, very lonely. When her sisters knew what had happened they mocked her and said that the little bird would never return. But he did, at last. And he sat on September’s shoulder and ate out of her hand and sang her the beautiful songs he had learned while he was flying up and down the fair places of the world. September kept her window open day and night so that the little bird might come into her room whenever he felt inclined, and this was very good for her; so she grew extremely beautiful.
Explanation:
 
When the bird was left on the window sill, it flapped its wings and flew into the air. But the Princess could not take this change of events lightly. She burst into tears. It was tough for her to choose the bird's joy over her affection for the bird. Soon after, the small bird fluttered out of sight, and the Princess seemed suddenly left alone.
 
Soon the news reached the eight princesses. They made fun of September and predicted that the bird would never return. However, contrary to the sisters' belief, the bird did return to September. The bird sat on September's shoulder and enchanted her with a few of the most beautiful songs learned while travelling across the world.
 
September began anticipating the arrival of the bird eagerly. As a result, she always kept the window open so that the bird could pay her a visit whenever he liked. Her pleasant connection with the bird proved beneficial for September, and she grew to be exceedingly lovely.
 
In this scenario, September choosing to prioritise the bird's happiness has played well for her. The friendship between the two grew beautiful, and so did September. We can associate this situation with the saying, "The face is the index of the mind". In other words, a peaceful mind reflects on one's face.
 
Meanings of difficult words from the paragraphs:
 
S.No
Words
Meanings
1
InclinedLikely or wanting to do something
Reference:
National Council of Educational Research and Training (2008). It so happened. Princess September - Somerset Maugham (pp. 34 - 44). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.