Cerebus is already feeling trapped, hating the rhetoric of the relationship with Joanne – she’s very mundane and he’s not. Also, she’s falling in love with him; I’m not sure what his feelings are, entirely. (I suspect he does have feelings for her and that’s one reason why he wants to run away.) He’s also counting down until Bear’s return (not sure why he has it decided that way, but he expects Bear to return at the end of the season.)