George and Lennie are the two main characters in Of Mice and Men by John Steinbeck, and even from the beginning we know that George has to take care of Lennie because he is unable to adequately take care of himself.
The two men have been traveling, and they stop near a deep pool near the river. George stops and wipes off his sweat, but Lennie
dropped his blankets and flung himself down and drank from the surface of the green pool; drank with long gulps, snorting into the water like a horse. The small man stepped nervously beside him. "Lennie!" he said sharply. "Lennie, for God' sakes don't drink so much. " Lennie continued to snort into the pool. The small man leaned over and shook him by the shoulder. "Lennie. You gonna be sick like you was last night. "
Obviously this is a common practice for Lennie, and George knows what happens when Lennie drinks too much water. It is a "green pool," so perhaps the water is not quite as fresh as it should be either, but it seems to be the amount of water Lennie is taking in that concerns George the most. It is a clear indication that Lennie needs George because he is unable to take care of himself.