In the chapter introducing Amortentia it's pointed out that each individual smells something different depending on what they're attracted by.
"And the steam rising in characteristic spirals," said Hermione enthusiastically, "and it's supposed to smell differently to each of us, according to what attracts us, and I can smell freshly mown grass and new parchment and -"
(Half-Blood Prince, Chapter 9, The Half-Blood Prince)
We know from JKR that Hermione smelt Ron's hair in the potion, though she wasn't prepared to admit it openly at the time.
Jess Mac: What was the third smell that Hermione smelt in the Amortentia potion in Half-Blood Prince (i.e. the particular essence of Ron)?
J.K. Rowling: I think it was his hair. Every individual has very distinctive-smelling hair, don't you find?
It makes sense in the scheme of things that she is attracted to something of Ron's. I also understand why parchment would attract her since she's a huge bookworm. But why the grass? Is there any indication about why freshly mown grass would inflame her passions?