Although the chemicals were causing romanti cinfatuation, they were also causing need and, in a young vampire's case - especially one who was still not fully aware of her own potential or, more importantly, limits - it was dangerous to hug.
Mainly, because Lucy's own return of the same now threatened not to let go. Being in full public view did not seem to matter a damn. Of course, more than a few others had already to begun to act similarly.
"Mmmm..."
Not purely the sound of human satisfaction, though. It was mixed with the beginnings of what sounded like a genuine purr.