During the 1914's trench warfare had developed, instead of what happened in the Battle of the Somme, Generals make precautions to protect their armies. Subsequently after France's counter-attack to regain the land lost to the Germans, Germany didn't want to be pushed back any further, so the Germans decided to dig trenches starting from the Swiss mountains all the way to the sea. It was the race to the sea.
An additional reason of why stalemate occurred was the superiority of defence. During the wars beyond the 1914 all the countries did was attack with either shells or running across no man's land with guns. The only defence was the barbed wire. However after the Battle of the Somme, Private George Coppard said that hundreds of the soldiers had died on the enemy wire. He also said 'The Germans must have been reinforcing the wire months. It was so thick that daylight could barley be seen through it'.
This showed that the Germans had been thinking defensively as well has attackingly. Another defensive based weapon is the machine gun, the British and the French underestimated it, but the Germans used it more to a greater affect against their Allies. After the attacks, which caused heavy casualties both forces made sure, they had an endless supply of machine guns and ammunition. The trenches were also a strong defence as they sheltered troops and kept their territory behind them. During the battle of the Somme the Germans trenches were re-enforced inside with concrete showing that trenches were thought of a defence mechanism.
The stalemate could have occurred by the fact that both Generals had a lack of military expertise. General Von Kluck and General Joffe had no back-up plans in case their original plans had failed. Also Generals had been using 19th century methods in a 20th century battle show their incompetence of the fighting a war. I know this because General Haig wanted to fight a battle with men on horseback showing his lack of skill of being a General.
Another potential motive of stalemate is that both Armies were closely matched, but not in size, but in strength, weapons, ammunition and artillery. Both forces had the same weapons of defence and attack such as machine gun, barbed wire and shell ammunition. None of the forces had the same amount of troops in their armies as Russia had the largest Army but was badly organised which gave the Germans a small chance. The French and British Armies combined was a big force but so was the German's and Austria-Hungary's. Both Armies had adopted the same method of fighting making it even more of a closer combat.
My conclusion is that stalemate occurred because, not one, but many reasons such as the incompetence of the Generals who didn't have a back up plan so had to make do with trenches, also the way the weapons and way of fighting have changed from all out attack to a defensive encounter, which the generals couldn't figure out. The most apparent reason I find is the how trench warfare had developed and the race to the sea, which showed that neither country didn't, was to lose any more territory.