In the book Of Mice and Men why does Lennie threaten to leave George?

Lennie threatens to leave George because George makes
Lennie feel like he is a burden. George can't have a girlfriend. George has to have the
life of a traveling laborer because wherever they go Lennie gets in trouble. Lennie has
the needs of a child like needing ketchup to go with his beans. Lennie plays with soft
dead animals because he likes how they feel even though they aren't healthy for him to
play with. Finally, Lennie says that he can go and live off in a cave somewhere by
himself and be just fine.

Lennie does this with intention.
He knows it will make George feel guilty. He knows George will never make him follow
through with it. But on the one hand, it is at least sweet that Lennie offers because he
does quite effectively drain George of his life. George can barely leave Lennie's
side.

Here is a good quote to
use:

Lennie
avoided the bait. He had sensed his advantage. "If you don't want me, you only jus' got
to say so, and I'll go off in those hills right there
-"