Why my feeling suddenly changed ?

Introduction to the characters : 

 Kyuhyun : he is an arrogant handsome guy and he like to tease everyone around him but at the same time he is the best lover and friend anybody can have because when it comes to the people he love he will be the most caring and thoughtful guy . He is friend with yemin and eanhae couple and his girlfriend ryeohyun who  lives with him in the dorm . He knows a little about his girlfriend brother ryeowook but not in personal but from his girlfriend who always talk about him. Can Love or date either girl or boy.

Ryeohyun: she is cheerful girl like to live her life freely the opposite of her brother and boyfriend who like to live their life calmly . She is bestfriend with eanhae and yemin couple .

Ryeowook : the cute and beautiful guy who look like girls in his appearance at the same time he is manly when it needed . He is bestfriend with heechul and yemin couple. Can Love or date  either girl or boy. 

Yemin : yesung and sungmin they are couple who love each other since they are kids and they are still together and promise to marry each other when they grow up . They are the realistic couple who take every thing with thinking and everybody goes to them to solve their problems .live with each other in the dorm.

Eunhae : donghae and eunhyuk they are couple who love each other madly since three years .they are the one's who make everyone laugh and have a fun time . live with each other in the dorm. 

Heechul : he is a very fashionable person . He works as a designer and he is a cousin for ryeowook and ryeohyun.

Siwon : a handsome guy who is every girl dream but he only loves one person but this person is taken and he will do anything just to have her.

Girls : sungmin and ryeohyun.

Boys: kyuhyun , ryeowook , donghae , eunhyuk , yesung , Siwon.