Why were soldiers on both sides most afraid of poisonous gas?

They didn't want to die

The use of poison gas was widely condemned and hurt German relations with the remaining neutral nations. The British roundly criticized the Germans with an extensive propaganda campaign, but gas usage continued to escalate throughout the war. The British rebutted in September of the same year with a gas assault at the battle of Loos. The shifting winds caused their own troops to suffer, with more British affected than Germans. As a result they switched to the use of artillery shells to disperse the gas assault with more accuracy.