It is always nice to see the happy part of what was a sad story. When first the northern kingdom, and then the southern kingdom were besieged and taken captive, to many it may have seemed like the end of the Hebrew nation. Only a few were left in the land and they were quickly being assimilated into the peoples around them. Those who had been taken captive had settled down in the lands of their captivity, and many were probably quite happy there.
But God intervened. He stirred up the spirit of Cyrus, and brought the people back to the land to build Him a temple.
Ezra is a book of renewal. It is a reminder that the purpose of chastening is never destruction, but restoration.