Her uber awesome voice.
Let me give you an example:
"Blue jeans, white shirt
Walked into the room you know you made my eyes burn
You were sorta punk rock, I grew up on hip hop
But you fit me better than my favourite sweater, and I know
That love is mean, and love hurts
But I still remember that day we met in December, oh baby!"
This is from 'Blue Jeans'. I love 'Without You' and 'National Anthem' too.
I don't know how to explain this but, her songs make me want to fall in love. Like NOW.