Disclaimer: My ownership of Star Trek ends with the transporter flashlight from the Frosted Flakes box. For shame.
For most of his childhood, Pavel Chekov kept to himself. His isolation was not intentional—in fact, he enjoyed the company of his four sisters and brother, his classmates and teachers. But as soon as Pavel learned to talk he knew he was different. As early as primary school he was already full of questions ("motor-tongue," his mother had affectionately dubbed him), and when nobody could provide the answers he sought them himself.
Pavel does not regret the consequential separation from his peers. Occasionally they were mean-spirited, but usually they just left him to his PADD and his star charts while they all partied, dated, and grew up. It is true that Chekov's rapid ascent through Starfleet accounted for an extreme deficit in his social awareness—after all, he was only fourteen when they finally let him enlist—but he finds that the other crew members find his lack of experience endearing.
In other words, they think he is cute.
Chekov feigns exasperation at this, but in his own way he is grateful for the acceptance. The crew does not question his abilities as he originally feared they would. They may perceive him as naïve or innocent, but they never doubt his knowledge or his dedication to the ship.
Still, they think he is simple. That his needs and interests are easily met, that he is uncomplicated. They see him as a blank corkboard, and every day they tack little pieces of themselves on him, defining who he will become.
Some of these pieces are bigger than others.