In the World of Ice and Fire: The Untold History of Westeros it is noted that King <PERSON> and <PERSON> were close friends as boys, along with <PERSON>. Being such good friends and a close confidant of <PERSON>'s, <PERSON> was became the youngest Hand in the seven kindgoms at the age of 20. Through the years as we are well aware, <PERSON> became paranoid and delusional, believing that his own son <PERSON> and close friends, most notably <PERSON> were plotting against him. There are several reasons why <PERSON> believed this.
1. His wife <PERSON>, although giving him an heir in <PERSON>, had countless miscarriages and still births, until the later births of <PERSON> and <PERSON>, while <PERSON>'s wife <PERSON> bore him two healthy twins, <PERSON> and <PERSON>.
2. After being kidnapped and held captive at Duskendale by Lord <PERSON>, who wanted more autonomy, <PERSON> believed that <PERSON> and <PERSON> had deliberately held back in freeing him, hoping he would be killed so <PERSON> could ascend the iron throne.
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<PERSON> were inherently jealous of <PERSON>, he hated how <PERSON> seemingly had everything that he didn't and at every opportunity sought to undermine and belittle him.
4. The greatest insult to <PERSON> came in 281 AC when <PERSON> appointed <PERSON> to the Kingsguard. As stated in the World of Ice and Fire: The Untold History of Westeros p121
Ser <PERSON> was also Lord <PERSON>'s heir, however, and carried all his hopes for the perpetuation of House Lannister, as his lordship's other son was a the malformed dwarf, <PERSON>. Moreover, the Hand had been in the midst of negotiating an advantageous marriage pact for Ser <PERSON> when the King informed him of his choice. At a stroke, King <PERSON> had deprived Lord <PERSON> of his chosen heir and made him look foolish and false.
<PERSON> couldn't refuse his king appointing <PERSON>, as this insult possibly would have cost not only him his life but caused the complete downfall of House Lannister, which is what occured to House <PERSON>. All <PERSON> could do was resign from his post as Hand and retire to Casterly Rock, which <PERSON> allowed him to do. 725