Why does george get angry at Lennie for?

When George told Lennie that they were going to have beans for the night, Lennie requested for ketchup once, with George responding that they do not have any with them.

Later on, while they were about to eat the beans, Lennie requested for ketchup once again, having forgot the first response from George earlier. This got George angry, stating that what ever they do not have with them is what Lennie always wants to have.

Lennie said that he did not want any ketchup, saying he wasw only fooling, but still cannot get to the point that they do not have ketchup with them. This also launched George into a long speech about Lennie's ungratefulness.