It actually was better developed in the manga. Hinagiku doesn't really fall for Hayate right after 3-4 chapter, but he does find Hayate as an interesting fellow first on. In their first encounter, Hayate saved a chick in a situation that resembled her situation where she was abandoned by her parent, so she's practically emotionally vulnerable by then and see Hayate as somewhat of a trustworthy saviour. Her first impression on Hayate is a really good one thanks to that.
Then [insert typical relationship progression between a boy and a girl which apparently in the age where they are highly interested in the opposite gender].
Hinagiku only certain she's finally falls for Hayate when Hayate help her overcoming her fear of height in her birthday.
I think that's 'better developed' than in the animu, but can't really say if it's 'well developed'.
inb4 "you're thinking into this too much faggot"