Rashi on Leviticus 10:6/רש"י על ויקרא י׳:ו׳

1ראשיכם] אל תפרעו] means, let not your hair grow long (cf. Numbers 6:5) (Sifra, Shemini, Mechilta d'Miluim 2 40). From this (from the fact that Scripture forbade these particular mourners to let their hair grow long) it follows that an אבל (one mourning the death of a near relative) is forbidden to cut his hair. Moses's words therefore signified: Ordinarily an אבל may not cut his hair but ye, disturb ye not the joy of the Omnipresent God by displaying signs of mourning (Moed Katan 14b).

2ולא תמתו LEST YE DIE — Consequently if ye do this ye shall die (Sifra, Shemini, Mechilta d'Miluim 2 41).

3ואחיכם כי בית ישראל BUT (LET] YOUR BRETHREN, THE WHOLE OF ISRAEL [WEEP] — From this it follows that for the miseries of the learned the duty devolves upon everyone (the entire community) to mourn (cf. Moed Katan 28b.)