@AsuraRaza very simple. because her father is missing so she thinks she doesnt deserve to be happy.
as a teenager myself, i can understand this sentiment. dont think too much about it - that is her reasoning, and if i were to answer a comprehension question on this manga, this is how i would answer.
i really like the 1980's of the us (where I live) and I adore the country of japan, and also rom coms so I thought this was literally the perfect manga for me and its so good cuz it makes me heart go oof