Finally you see that Cosette tells Marius that she is leaving with her Jean which remembers me of a song by one of my favorite bands called Boys Like Girls with their song the great escape:
Throw it away
Forget yesterday
We'll make the great escape
We wont hear a word they say
They don't know us anyway
You begin to see that Cosette is becoming a woman and is truly falling in love and her father disagrees with this love, making them forbiden lovers. This reminds me of a song by one of my favorite artists John Mayer called daugters although she did not have a mother: 
Fathers, be good to your daughters
Daughters will love like you do
Girls become lovers who turn into mothers
So mothers, be good to your daughters too
Cosette and Marius fell in love, it was like love at first sight, the start of almost every love story. The part of the song Love at First Sight by Kylie Minogue that goes perfectly with this is:
It was love at first sight'Cause baby when I heard you
For the first time I knew
We were meant to be as one
Was tired of running out of luck
Thinking 'bout giving up yeah
Didn't know what to do
Then there was you
And everything went from wrong to right
And the stars came out to fill up the sky
