The comment Shima stopped himself from making was 100% legitimate I feel like. Yes Shima HAS fucked up in the past, and yes Mukai helped him to figure it all out, but in this case Mukai was being a bit judgmental due to his pent up frustration.
As a response to "You don't actually give
any thought to how others feel," biting back a textbook example of the perfect solution fallacy is… frankly for the best. Mukai wasn't saying he should have people figured out, he was saying it was an easily-discernible faux pas to ask someone whose romantic feelings he rejected recently enough for people who were paying attention to remember to associate with him as though those feelings and rejection wouldn't immediately color any refusal she might offer.
We've had this shit figured out at least as far back as Descartes: it's impossible to have full knowledge of
anything as long as conscious existence is subjective. You
have to make assumptions, to some degree, but what people will generally get on you about is making
unreasonable assumptions. Social relationships can be tricky, but Mukai was expressing frustration with what appeared to be a lack of any effort at all and Shima's knuckled-down first reaction was "It's impossible for me to be perfect at it so it would be arrogant of me to try" in an area of practice where good-faith effort gets you a lot of goodwill.