Without your schemes my life, it seems, is empty
I spent all my time keeping you from doing wrong.
You were my only nemesis
I'd foil your plans, but still I miss
The moments when we didn't get along
I fell for you the moment that you pushed me
You captured me and held me for so long
But now you're doing battle with a panda from Seattle
I miss the moments when we didn't get along.
So search your heart, please, Doctor D
And I am sure that you will see
That you were always meant to be
My only lifelong enemy
You gave my life heroic cause
Back when my only mission was
The moments when we didn't get along.
Ooh.

http://phineasandferb.wikia.com/wiki/When_We_Didn%27t_Get_Along