Juliet wants the bird to be the nightingale instead of the lark because the nightingale only sings at night, while the lark sings in the morning. If it's the nightingale they heard, that means Romeo can stay with her a bit longer, but if it's the lark, then Romeo has to leave.
Explanation: The setting affects the characters-therefore affecting everything about them. It can not only change their personalities, but it can also change the way they view things, etc. !
