Why did the swallow stay with the Happy Prince?

In the story, Swallow is on his way to Egypt before he stops to spend the night at the City, and that is where he meets the happy prince. The prince begs the swallow to stay with him for one night and act as his messenger. After giving away the two sapphires of his eyes, the happy prince became blind.

The swallow was moved by the happy prince’s kindness and his great compassion towards the poor, he decides to stay with the prince always rather than flying to Egypt to spend the winter with his friends. Hope this helps.