Why must I bring you your lunch with 〜なければならない

Grammar Explanation: Must do with 〜なければならない

Spinel Sun calls out to Ruby Moon at school. After a brief discussion, Spinel Sun gets to the reason for his appearance there.

スピネル・サン:: 「で」; “So.”; 「どうして　わたしがあなたの忘(わす)れたお弁当(べんとう)を届(とど)けなければならないんですか」; “Why do I have to deliver the lunch you forgot?”; Literal: “Why is it that I must deliver the lunch you forgot?”
ルビー・ムーン:: 「だってー　せっかくエリオルが作(つく)ってくれたんだもーーん！！」; “Well, because Eriol went to all that trouble to make it for me!!”

Key Points

1届(とど)けなければならない = must deliver

The verb is 届(とど)ける “to deliver”; dropping る gives the stem 届(とど)け before なければならない.

2Why this feels like “have to”

〜なければならない literally comes from “if not, it will not do”, but in use it means someone must do the action.

3Parsing あなたの忘(わす)れたお弁当(べんとう)

忘(わす)れた modifies お弁当(べんとう), so the phrase means “the lunch you forgot”.
The あなたの marks whose forgotten lunch it is, not possession in the sense of ownership alone.

4Polite complaint with んですか

どうして … んですか asks for an explanation: “why is it that…?”
Combined with なければならない, it sounds like a formal but pointed complaint: “Why do I have to…?”

弁当 (Boxed meal)

弁当(べんとう) is a meal that is packed in advance into a container so it can be taken along to eat while out and about, at work, or at school.