It partially shows why in the first chapter, but doesn't say it outright, It seems like he broke up with her, probably due to insecurities about himself
Yeah, hey bro maybe if you had AT LEAST ONE positive interaction with sorata, instead of only ever treating like he's a used condom on the side of the road, he'd might feel a little better about himself
No they should not come with, it's an undiscovered dungeon, for all they know every monster could be s rank, but they're bringing two noobs who don't have proper experience in imminent death scenarios