that's why

yes
i am surely supposed to be happy
but at this moment i can't be

and soon i know why ...

it's because someone fails to share the supposed happiness with me,
or maybe because she's not optimistic enough to sense that kind of happiness ...
while, at the same time, her heart is one of the few essential factors in this world constituting my happiness

so i can't be as happy as i am supposed to be

it's the simple logic