Hi Laura,
왜 이렇게 좋아하는 거야? = 1) why do u like it/him/her/them/me so much?
2) Why are you so happy? (Sometimes it is used when asking why someone is laughing.)
Ex. A: 휴.. 걔가 너무 좋아서 미칠거 같아 (phew.. I'm almost going crazying because I like him so much).
B: 왜 이렇게 좋아해? (why do u like him so much?)
A: (laughing) bahahahah
B: why are you so happy?
Hope it helps.