According to what she says in the manga, she first became attracted to him through the Newspaper Club's newspapers, figuring out that he had the nature of a loner like herself, which was true to a degree. Later, she likes him even more as he saves her from the Kraken guy,Otsubo or something like that, and becomes her first friend. This turns into love after being with him for around two years.
It started out as the kind of love you would imagine from any stalker, a blank idealization on someone they only know to the slightest degree. Technically its always been love, but her experiences through the two years as well as their adventures has refined it as she actually got to know him.
This is a harem manga, a bunch of girls getting in a position where the guy does something special for them causeing them to fall for him is common in harem manga. For Mizori, she was a loner, after sympathizing with Tsukune who wrote like he was weak, she gained an intrest, she like many snowwoman of legend fell for him (so it's a species thing). After he becomes one of her 1st friends and rescues her and helps her make more friends and constantly risks his life for her and her friends, she couldn't help but love him. But it all started with her speicies thing of gaining an intrest in a man and freezing them (read about the Yuki-onna).