In the beginning of the Mahabharat war, we see that Arjuna became depressed and dejected seeing his own kinsmen in the opposite side. Therein we find that Lord Krishna encourages Arjuna by again and again telling Arjuna, "Stand up and fight! O Bharatha!" These words of encouragement are a source of great strength. On the other hand we see that Karna had Shalya as his charioteer and he was always discouraging Karna. King Shalya was the brother of Madri (mother of Nakula and Sahadeva), as well as the ruler of Madra-desa or the kingdom of Madra. The Pandavas had counted on Shalya joining their side with his huge army. On the way to assist the Pandavas in the Kurukshetra war, Shalya was tricked by Duryodhana, who arranged a huge feast for Shalya and his men. When Shalya was impressed by the hospitality of his host, whom he mistakenly thought to be Yudhisthira, he offered to be at his host's service. Unable to turn down Duryodhana's request to join the Kauravas, Shalya met Yudhisthira and apologized for his mistake. Yudhishtira, knowing that Shalya was a great charioteer and forecasting that Shalya would someday be asked to be the charioteer of Karna, extracted a vow from Shalya to the effect that Shalya will demoralise Karna and dampen his spirits. Shalya assured the Pandavas that he would do the needful.
So during the war, when he was serving Karna he told, "Do you seriously you think you can bring down Arjuna.....A rabbit can never bring down an elephant!" At the end, Karna pulled out his Nagastra and aimed it straight at Arjuna's head and was about to fire it, when he was interrupted by Shalya. 'Karna! Do not aim for the head....Aim for the chest!' Shalya shouted. Even though Shalya was giving proper advise at this point, since he had continuously demoralized and discouraged Karna, Karna was in no mood to heed Shalya's warning. Shalya left Karna in the lurch since Karna did not heed his advice. Karna probably thought that Shalya was still trying to distract him by degrading him and fired the nagastra straight at Arjuna's head. Arjuna was not quick enough and could not defend against it. Krishna saw the astra coming as if in slow motion and right at the last minute when the weapon was almost among them, he pushed the chariot down and so....the weapon which came for the head went with the crown. 'Use the weapon again!' A voice came to Karna. Karna saw astonished as the astra looked at him in the shape of a snake. 'This Arjuna has killed my family in the Khandava forest...Use me again and this time I shall kill Arjuna!' Karna looked angrily at the astra. He said 'The only person I trust is myself....I do not trust anyone else to fight for me! Go away!'
Thus continuous discouragement made him lose trust on everyone. That is why we see that the children who are always discouraged in their young age, grow up to be highly negative personalities with no love and trust on anybody.