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.