Why Snape's worst memory?

[vmonte]

No: HPFGUIDX 137330

Marianne wrote:

As for the prank, it is a pet theory of mine that the prank
happened much later in their school career, perhaps 7th
year. A while ago I posted that I think that Severus loved
Lily and, even if she didn't return or even know his feelings,
he was at least comforted by the fact that Lily hated James.
But, again in my theory, it is the very fact that James saves
his worst enemy's life that gets Lily to change her mind
about him, so not only does Snape owe James a Life Debt,
but he loses the girl he loves to him. I put this event in their
7th year because that's when Sirius and Lupin said that
James and Lily started going together. Nevertheless, I won't
mind being proven wrong; I just hope that exactly
what happened during the prank (as well as what happened
at Godric's Hollow and why Dumbledore trusted Snape) are
three memories I would like JKR to show us in book 7.

vmonte:
Why would JKR mention that Harry noticed that Lupin looked strange 
and that he then wondered if the full moon was approaching if it 
wasn't important? Why add this info at all? 

Vivian