What Luo An is doing now is building empathy with the other party.
The information about Frederick that Mona sent him shows that Frederick never attended high school, indicating he has a low level of education and is unlikely to have studied psychology.
Having no family made him lack affection, which is why he could commit murder and dismemberment without any psychological burden.
Frequently changing jobs suggests either he was clumsy and unable to do a good job and thus always got fired, or he was dissatisfied with his job, such as disliking the job for being too much trouble, not getting along with colleagues, or the boss always picking on him, etc.
But regardless of which, it's enough to say that Frederick does not like the act of "working" itself.
Therefore, complaining about work would definitely resonate with Frederick.