It’s low self-esteem, Okarun doesn’t think it’s possible that someone would actually want to talk to him so he refuses to believe obvious signs to the contrary.

Side note: it’s a huge pet peeve of mine when people use “introversion” to describe behavior like this because it’s not what “introversion” means at all and because it’s conflating maladaptive behavior which you could change with an inherently personality trait which you can’t change. Being introverted just means your preferred balance of time alone or with small groups (one or two close friends) vs time in large social groups is skewed more towards small groups. It doesn’t mean that social phobia/paralyzing social anxiety which causes you to completely isolate yourself 99% of the time is “just how you’re supposed to be”. If this is how you feel, you have legitimate problems which are fixable, you aren’t doomed to suffer in loneliness forever.

I can relate to this scene, but in a different way. I think Okarun was so hesitant to talk to Momo for several reasons; lack of self-confidence, protecting himself from rejection, social anxiety from a school career of harassment, etc. All of this led to Okarun thinking that he doesn’t deserve a friend like Momo, so of course she will ignore him, so might as well ignore her first.

In my younger, school-aged years, I very likely would have walked past somebody that I spent time with the day before, but for a different set of reasons. Growing up, my family moved almost every year due to my father’s job. I went to >10 schools before going off to college. So, I just got in the habit of not really investing any effort into making friends, since I was just going to have to leave them anyway (pre-internet times). I am quite social and don’t suffer the same set of hang-ups that Okarun has, but I very much can envision myself not talking to someone at the school gate even after spending a whole day with them.