why do harry and hermione does became a couple?

... No they don't...

THEY don't 吻乐队（Kiss） -- it's an apparition, tempting Ron. It's not the real Harry and Hermione.

exactly sapiencia. SPOILER: they were just 图片 coming out of a horcrux to harass ron, they weren't the real people ;) and even after that situation was over, harry even told ron that him and hermione only loved each other in a brother/sister relationship :P