Do you guys remember when Fiona's permanent Ogre figure was activated by Shrek's kiss in Shrek? It had that amazing transformation effect! see:
In Shrek: Forever After, why was it passive?
The magic of True Love's Kiss is only applicable the first time around:
Princess Fiona: But... how will you kiss me?
Shrek: [bangs his head] What? That wasn't in the job description!
The Donkey: Maybe it's a perk!
Princess Fiona: No, it's destiny! You must know how it goes! The Knight rescues the Princess, and then they share true love's first kiss...