In Of Mice and Men, what is Crooks somtimes afraid of late at night in his little room?

I would disagree with that.  I think Crooks is very concerned that when his life is over he will not have made any real connections with anybody.  These are his lines:

 "I didn't mean to scare you.  He'll come back.  I was talkin' about myself.  A guy sets alone out here at night, maybe readin' books or thinkin' or stuff like that.  Sometimes he gets thinkin', an' he got nothing to tell him what's so an' what ain't so.  Maybe if he sees somethin', he don't know whether it's right or not.  He can't turn to some other guy and ast him if he sees it too.  He can't tell.  He got nothing to measure by.  I seen things out here.  I wasn't drunk.  I don't know if I was asleep.  If some guy was with me, he could tell me I was asleep, an' then it would be all right.  But I jus' don't know."