Why did it bother her so much? So Parvati and Seamus had a nice date together, shouldn't she be happy that her best friends were hitting it off? Lavender didn't want to entertain the notion that she wanted to date Seamus. If she did, she would be dating him already.
They had one date. One kiss. Maybe more than one kiss, but that did not mean they were going to spend the rest of their lives together. No, Lavender just didn't want to depreciate in importance to either one of her best friends. She did not want to be replaced. It was a pity she could think of no nice way of wording her worries, because it left her with little platitudes and false enthusiasm.
"That is so weird," She said, but with a thrill of giddy excitement, "Seamus! I can't believe you kissed him. That was brave. Who knew you were such a Gryffindor after all?"
Lavender grinned at her best friend, trying with all her might to just be happy for her. It would be what she would want, if the roles were reversed, "Don't forget me when you're married and having little Irish babies with beautiful complexions."