Why did Lear decide to leave Goneril's castle?
Having disowned his youngest daughter Cordelia, Lear divides his entire kingdom between the other daughters, Goneril and Regan, with the specification that he can live with them alternately along with his retinue of one hundred knights. Lear wants to be free to enjoy himself in his old age.
First he moves into the palace of the Duke of Albany who is Goneril's husband. In the opening of Scene 3 of Act 1 we see that Lear and his hundred knights are creating an uproar in Goneril's household. They do nothing but eat, drink, and go out hunting. Goneril tells her steward Oswald:
By day and night he wrongs me; every hour
He flashes into some gross crime or other,
That sets us all at odds: I'll not endure it.
His knights grow riotous, and himself upbraids us
On every trifle. When he returns from hunting,
I will not speak with him; say I am sick.
If you come slack of former services
You shall do well; the fault of it I'll answer.
At that point there is the sound of hunting horns offstage--and we can imagine Goneril's body language when she hears her father and one hundred knights returning on horseback covered with dirt and carrying the bloody carcasses they have killed, probably accompanied by a pack of barking hounds. It is like a housewife's nightmare.
With Goneril's encouragement, Oswald treats Lear coldly and provokes a quarrel between the King and his daughter. She lashes out at him, in sharp contrast to the overwhelming love she expressed when he was giving his kingdom away. The quarrel becomes so heated that Lear, in Act 1, Scene 4, shouts:
Darkness and death!
Saddle my horses; call my train together.
Degenerate bastard! I'll not trouble thee:
Yet have I left a daughter.
Lear does not realize that Regan doesn't love him any more than Goneril, but he finds out very soon. Both daughters have an understanding that they will disregard the conditions under which they obtained their halves of their father's kingdom. Both of them tell him they will only accommodate him and no longer provide for his one hundred knights. These two shrewd women realize that Lear could become a definite threat now that they have provoked him and shown their true colors, and they are justifiably afraid that he might use his one hundred knights to reclaim his throne.
Lear ultimately finds himself homeless and destitute, since he is too proud to accept his daughters' grudging hospitality and become a meek dependent, almost like a charity case. He would even lose his Fool, because that unwelcome man has the bad habit of telling the plain truth.