Yiftach refused to be a leader for Klal Yisroel unless his was a permanent leader even before he went to war against Amon. (Malbim). His brothers were Rodef him and disowned him, and only when they needed him to fight Amon did they suddenly soften and ask him to be a leader just for this battle itself. But he insisted to be a total leader right from the start.
They had been cruel to him, and in order to avoid Machlokes, he ran away from them. But now he put them in their place, telling them: "Only when you need me you come to me?" Especially Yiftach who was an Am HaAretz and a: קַ ל שֶּ בְ קַ לִׁ ים lowest of the low, needs to assert himself or else he won't be successful. (R' Chaim Shmuelevitz Zatzal).
The Seforno in last week’s Parsha says
If your nature is sunk in lowly Shiflus, with: “I’m a zero” repeated in your head overtime, you need overtime of the opposite: “I’m Gevaldig!”
Moshe told the Levi'im that your Avoda in the Mishkan is done Davka in front of Klal Yisroel in order to highlight their Chisaron that you were chosen instead of them. It appears that a Ben Levi can feel Ga’ava since he was chosen over the Bechorim (firstborn). Every Yid needs to know his superiority over others – but there is a limit to this.
The problem of Bnei Levi is they wanted still more. Ahron was also told by Hashem regarding the Nesi'im: שֶּ לְ ךָ גְ דוֹלָ ה מִׁ שֶּ לָ הֶּ ם “You have more than them”. There is nothing wrong with kosher Ga’ava and appreciating your superiority over other Jews. You are giving Hashem credit, and this is not Ga’ava.
Especially a person like Yiftach who was so rejected at first, needs to Davka assert himself. It is interesting the Shevet Dan, the: יָרוּד שֶּבִׁשְבָטִׁים lowest of the Shevotim, was the only Shevet that named their main city “Dan”: כְשֵׂם דָן אֲבִׁיהֶּם יהושע יט מז after their ancestor Dan – Davka the: יָרוּד lowly one is proud!
