And from the same episode that song comes from, the part where Perry leaves.
watching Phineas give up hope on the island in Summer Belongs to You.
"The City of Love". It's the culmination of how many seasons of Isabella's frustrations and Phineas' Freaking.Obliviousness. Her plaintive little voice, and the fact that he's still oblivious as she sings this right in front of him are just sadness on overdrive.