It is a psychological event called "love on first sight".You don't even know that person very well but something attracts you to that person...it is like your inner space is controling you and it knows that this person is meant to be with you.It happens very frequently between teenagers and between adults too,but between teenagers it has a really fast ending but sometimes between adults it can be love for whole life.
And between Robert and Giselle...it's hard to say when the chemistry started working...but I think it was...maybe in that "Angry" scene.

Yeah I was going to say that too cause there was a slight moment of silence and Robert was going to kiss her but he said Ok and left and Giselle was speechless.

And you could feel the electricity in air.