Rashi on Numbers 11:29/רש"י על במדבר י״א:כ״ט

1המקנא אתה לי means, “art thou envying where I should envy”.

2לי — the word לי meaning the same as בשבילי, “for my sake”. Wherever an expression of the root קנא is used it implies that a person sets his heart on the matter, whether it be to take vengeance or to help; — emportement in O. F. (English = zeal) — he holds the thickest (heaviest) part of the load (i.e. he takes the responsibility for carrying out a matter).