When King Lear speaks the words “O, reason not the need” in Act 2 of the play that bears his name, he is making a statement that principle and kindness are sometimes more important than reason and need. The issue here is Regan and Goneril's treatment of their father. The sisters have joined forces against Lear now that he has given them their inheritance. They no longer need to treat him with a fawning love. Rather, they can make demands and threats, which is exactly what they do.

Lear always travels with a large retinue of knights. Certainly he has more men than he strictly needs, but he has always kept them with him because they are part of his support and pageantry as king. Regan and Goneril now want to deprive Lear of his retainers if Lear wants to stay with either of them. At first the sisters insist that he cut the number of knights by half, but after a long argument, they bring that number down to none. If Lear wants to stay with his daughters, he must give up his servants. But to give up his servants basically means giving up any outward evidence that he is king. He might as well be a beggar in his daughters' homes.

Therefore, Lear cries out “O, reason not the need” after Goneril asks him why he needs so many knights. It isn't about the need for them. Lear doesn't “need” them; he wants them. They give him purpose, and they show that he is still the king. The issue isn't about reason and facts. It is about principle (that Lear still has authority) and kindness (that his daughters act with love and respect toward their father and his desires). When Regan and Goneril remain steadfast, Lear rushes out into the storm.