As mentioned, a reason for McClellan is that he was a very paranoid person. Woodworth says that, “McClellan, probably driven by a compulsive fear of failure, consistently accepted the highest, wildly unrealistic figures for the enemy’s numbers.” For this reason, McClellan was extremely paranoid and made up exaggerated numberd as an excuse maybe to not use his numbers and also he did not want the blame for the many deaths of soldiers
McClellan was just hesistant in nature. According to Steve Woodworth, “McClellan was beginning to exhibit one of his greatest flaws as a general, a fundamental lack of nerve.” McClellan, for some reason, always presumed he was always outnumbered and therefore refused to use his reinforcements. In reality, McClellan always had the superior number of forces. He could have crushed Lee at Antietam but prolonged the war by being so conservative and hesitant to act.
McClellan had a stressed and soured relationship with Abraham Lincoln. Lincoln is quoted as saying to let him “borrow” the army if McClellan had no plans of using the army. Lincoln grew tired of how conservative McClellan was being and given the fact that he could have ended the war and destroyed Lee at Antietam but he did not pursue Lee and the war trudged on. Ultimately Lincoln had enough of McClellan, who lacked the guts and agressiveness that a general needs, and replaced him with Burnside. The turning point of the strayed relationship between the two men was when McClellan got diagnosed with a case of Typhoid fever and did not have any plans to move the army or to pursue the
Confederate army. It is believed that the reason that Lincoln did not approve of McClellan running the army I is because he was so hesistant and could have
prevented many deaths and ended the war. If McClellan
wanted to prevent deaths of his army did he not pursue
and destroy the Confederates when he had multiple
Source: http://www.anb.org/articles/04/04-00674.html?a=1&n=george%20mcclellan&d=10&ss=2&q=4- American National Biography