I love you too
A rare, literary, and euphemistic response to a confession of love, originating from a Meiji-period translation by Futabatei Shimei. Sometimes used as a reply to 月が綺麗ですね (the moon is beautiful, isn't it?), which itself is a famous indirect confession. The literal meaning is 'I wouldn't mind dying (now)', implying that hearing those words has made life complete.
See also: 月が綺麗ですね
"The moon is beautiful, isn't it?" "I love you too."
In this context, '死んでもいいわ' means 'I love you too.'