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 -"  
<sub>*(Half-Blood Prince, Chapter 9, The Half-Blood Prince)*</sub>

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?