Answer :
1. lonely - deserted, isolated, desolate...
2. fluttering - flapping, flopping, wiggling...
3. twinkle - glimmer, shine, flicker...
4. sprightly - vivacious, bouncy, cheerful...
5. glee - delight, elation, joy...
As for why the poet didn't use these synonyms instead, I don't know. The answer may be simple - he may have preferred the words he wrote, or maybe those words fit the structure and rhyme better.
2. fluttering - flapping, flopping, wiggling...
3. twinkle - glimmer, shine, flicker...
4. sprightly - vivacious, bouncy, cheerful...
5. glee - delight, elation, joy...
As for why the poet didn't use these synonyms instead, I don't know. The answer may be simple - he may have preferred the words he wrote, or maybe those words fit the structure and rhyme better.