AHH OKAY YAY so there quite a few reasons so i will list them and yap unnecessarily in the process!
1. basically when onaip gets back from the SS-Class dungeon they find this device that allows a guide to guide somebody without physical contact. Andante is SO stupid for this, he literally hires a new guide to test out the device but HE DOESNT TELL HYOSEOP WHY!! nor does he tell the public either so there is so many rumors speculating. And when the new guide meets Hypseop she asks him if Andante is good at.. um yk.... So like OBVIOUSLY Hyoseop feels uneeded and stuff.
2. People (especially guide yoojin and shin haechang) keep telling Hyoseop stuff that makes him feel bad and like Andante doesnt love him and stuff i would talk more about this but i dont remember exactly what they say.
3. in the SS-Class dungeon they go to retrieve the body of the ex guide of onaip. It is shown on the television Andante hugging this corpse very lovingly. It is actually Andante's BROTHER, but Hyoseop didnt know and Andante just assumes he knows. There is also a lot of articles and stuff saying that the body they went to retrieve was Andante's past lover!! Hyoseop finally realized why should he stay if he is just being used. (He doesnt know that andante's feelings are genuine now)
4. Hyoseop has a past trauma about Espers becoming unhealthily obsessed with their guides and mistaking it as love. Andante shows a lot of possessive traits that triggers a trauma response from Hypseop and so he wants to leave out of fear.
OKAY THATS ALL!! this should save you some agony from cliffhangers and weekly chapters im excited to see this stuff drawn tho

If the purple haired guy doesn’t back off, I’ll raise hands.
And my baby, I understand he’s in love, I hope the blonde dude truly cherishes him still after the mission, or imma throw hands as well.
If not, the MC deserves someone better boo.