Why から is used instead of は/が?


I found several から quite confusing for me.

The context is about a boy and a girl, and she's trying to ask him out, but
she eventually can't do so, and asks him to meet the next day:

> Girl: あ…明日も会ってくれる？
>
> Boy: やっぱ俺 **から** 言うか --> Why から is used instead of は/が ?
>
> Girl: あ…明日は言う **から** --> Which use has から here?
>
> Boy: ああ いいよ。でも、その前に 俺 **から**...... --> Why から is used instead of は/が ? Which
> are the omitted words?
>
> Girl: 本当に？会ってくれるの？




Without further context it is hard to say whether my answer is completely
correct, but considering the anime's theme, I'm pretty sure I got the right
idea.

Am I correct in assuming they are both trying to confess their love to each
other? If that is the case then both of the から used by the boy are the same,
and indicate a direction from which something will be said (in this case, a
confession of love from his behalf). A common anime trope is to wait for the
other person to confess their love first, but as he doubts she will ever do
this, he resigns, saying that it will have to come from (から) him.

The girl's use of から is a bit more obscure without further context, but as she
explains on her dialogue immediately following what you posted, she seems to
have "forgotten" what she was gonna say. Therefore, she is requesting they
meet up tomorrow, saying she wants to do so because (から) she will certainly
say it tomorrow [what it is she forgot].

