Vulnerability because of vanity

The predisposition of characters to believe things played a huge part in King Lear. Had Lear recognized the true nature of his daughters and Gloucester the nature of his sons, none of the other events in the play would have occurred.
First, King Lear in his growing senility wanted to believe that all of his daughters loved him more than anything else. His predisposition to believing this, along with his vanity, led him to take Goneril's and Regan's praise as absolutely true, and Cordelia's truthfulness and sincerity as a lack of affection. Because he already thought that they loved him so much, he was more prone to believing Goneril and Regan were speaking truthfully. Thus began Lear's downfall as he handed over his entire kingdom to the two of them, believing that they would care for him-- another assumption that he convinced himself was true through their saccharine professions of love for him.
Gloucester, having heard from Kent so recently how wonderful a boy Edmund was, was especially vulnerable to the lies Edmund told him. He said that Edgar was trying to kill Gloucester to take his wealth, and Gloucester, wanting to believe that his bastard son was a good person despite being a bastard, believed him. Instead of saying something along the lines of "You're an evil person, my son would never do such a thing," he took Edmund's word on the matter. This ultimately led to his blindness as he put his trust in the wrong son and let Edmund do as he pleased while he was focused on Edgar's supposed treachery.
This goes along with Edgar's willingness to believe that his brother is telling the truth and that his father is angry with him. It could be said that Edgar is just a complete idiot who doesn't understand what's going on and blindly does what he's told to, but his assumption of the Poor Tom character implies that he sort of knows what's happening and that he really did trust his brother.
This brings up an interesting question-- why is it that everyone in the play, except for Cordelia, seems to trust the 3 main evil characters? People in general out of vanity seem predisposed to believe positive things that someone says to them, and negative things that someone says someone else said about them. The beginning of this play is riddled with this theme, as Lear, Cordelia, Gloucester, and Edgar are all betrayed by smooth-talking but insincere family members.
--Little Red