The Netziv is answering the smichas parshiyos of why ???? ???? is next to ?? ?????. He’s saying that it’s the reason you can’t add a negative mitzva out of Yirah. So the reason you can’t add a lav is because of ?? ?????, and the proof is the smichas parshiyos to ???? ????, which is incompatible with adding too many lavin.
I hope I’m understanding it correctly from my high school Chumash.