Haven't you heard of love? Gosh, SO many people ask this question! It's like asking why is the sky blue. She just did! She started liking him upon first seeing him at King's Cross in 1991, partly because of his celebrity, and she grew so embarassed around him the netx year at school she couldn't speak. He was nice to her, but didn't reciprocate. She was still shy (even more so considering he had just saved her life) in PoA. But in the summer before GoF, while sharing a room, Hermionie told her to relax around him and be herself and maybe even date other guys. If you've noticed, that's when she started treating him like a friend. And at the Yule Ball with Neville (she had just missed a chance to go with Harry, her crush, but didn't turn back on her promise to Neville even though it was just platonic), she met Micheal Corner and they started dating at the end of the year. Their relationship went almost all through the next year (during DA meetings which she brought him to he wouldn't-or maybe couldn't-hex his girlfriend) but she dumped him because he was being a sore loser about Gryffindor beating Ravenclaw in the quidditch cup. Meanwhile, Harry and Cho had broken up because she was jealous over Hermionie and grieving over and still sort of in love with Cedric even though he was dead. So anyway, Ginny dumped Michael and immediately began dating Dean Thomas, Harry's roomate. At first, he didn't give it much thought, but he wanted to hug her more at the Burrow, and when he smelled the love potion Amortenia which smells like the the thing the indiviual is attracted to, he absent-mindedly smelled her hair. Soon, when he and Ron found her and Dean kissing in a hallway and Ron got furious, it lead to a falling-out between the two siblings, and Harry realized he wanted to rip Dean limb from limb. He tried to pretend those feelings were strictly "elder brother-y" but he was having "intimate" dreams about Ginny making it glad Ron could not perform Legillamency. He and Hermionie conforted each other of Dean and Ginny and Ron and Lavendar (Ginny still mad at Ron and calling him a filthy hypocrite). Ginny's feelings for Harry were starting to resfurace and she didn't care much about seeing Dean after the holidays and the fought a lot because of his over-proteciveness. And when Harry was under the influence of Felix Felicis, the lucky potion, he was under his invisibility cloak in the potrait hole and bumped her, but Ginny thought it was Dean, and this lead to a huge fight and their break-up. Then, after they one a Quidditch match, in the common room celebrating, Ginny randomly kissed Harry in front of everybody (in the movie alone in the Room of Requirement while hiding his Potion's book). Ron had an expression that said "I don't like it but I'll deal" and Dean was so mad he broke a glass, but didn't have resentment towards Harry. They began dating and he found it "like something out of someone else's life" even though because of his detentions they didn't spend a lot of time together, but after Dumbledore was killed, he broke up with her at the funeral because he was worried as they went public Voldemort would harm her to get to Harry, as he'd done before when she was just his best friend's sister. She protested, but agreed, admitting she had never really given up on him. Once they met again at the Burrow that summer, it was awkward,but then on his birthday she "couldn't think of what to get him" and they were talking about "dating oppurtunites" when he was doing what he was doing so she kissed him. He was lovin' it, but Ron and Hermionie interrupted accidentally and awkwardly. Ron got pissed for him toying around with her because he had just dumped her and she was pretty “cut up about it” and now he’s kissing her. She winked at him at Bill and Fleur’s wedding when her aunt said he dress was too low-cut. Also at the wedding, after finding out Ron and Hermione were “sort of” together, Viktor Krum tried to go after Ginny and Harry (in disguise as Barney Weasley) said she has a jealous and big boyfriend. They had to leave immediately when death-eaters invaded so he never said goodbye and was wrapped up in horcrux stuff, but he did stare at her name all the time on the Marauder’s Map, hoping she knew, and got excited at the thought of her being near at the burrow. They talked at the battle of Hogwarts, and when he was about to sacrifice himself , he was her consoling a little girl but he was under the invisibility cloak so she didn’t see him, but he felt she “sensed his presence.” She was the last thing he thought of when he sacrificed himself to Voldemort. But once he defeated Voldie, the danger was gone and they were free to resume their romance. From sometime between 1998 and 2017, they got married and had three kids, James Sirius, Albus Severus, and Lily Luna. So to answer your question in a basic way, she first fell for him for immaturely because of his celebrity, but he soon turned into the love of her life, and because she began acting normal and making him jealous, the same occurred for him. They are equals and love each other just as much as anyone in the series.