While Vegeta has come a long way since his initial antagonistic introduction at the start of Dragon Ball Z, it's safe to say the Saiyan Prince is still a pretty arrogant guy, largely motivated by a deep sense of pride and unquenchable desire to be the best. Vegeta's usual hubris would endanger the entire Dragon Ball Universe during the Cell Saga, when the Super Saiyan confronted the synthetic villain for the first time.
Though Vegeta's disastrous handling of Cell would cost the Z Fighters dearly by the end of the storyline, this blunder was not only a defining moment for Vegeta but underscored the reason why so many Dragon Ball fans love the Saiyan Prince.
The villainous Cell was introduced at the very end of Dragon Ball Z's Android Saga as the evil Doctor Gero's ultimate weapon and means to avenge his death at the hands of Androids 17 and 18. With both Androids destroyed in Cell's native timeline, Cell acquired Future Trunks' time machine to travel to Dragon Ball's present day to absorb them and reach his perfect form. After besting Piccolo, Cell absorbed Android 17 to transform into a semi-perfect state and cornered Android 18 just as Vegeta and Future Trunks caught up to him, after both warriors trained for a relative year inside the Hyperbolic Time Chamber. Easily besting Cell in his current form and believing he had transcended the level of Super Saiyan, Vegeta allowed the villain to absorb Android 18 and reach his perfect form -- only to be shocked when Perfect Cell outclassed him in every conceivable way.
Vegeta allowing Cell to absorb Android 18 is the greatest mistake he has made in the franchise to date, with Cell nearly destroying Earth and the resulting final showdown ultimately causing the deaths of Android 16, Goku and Future Trunks -- albeit only temporarily for the two Super Saiyans. And yet, the move was certainly not without precedent: On Namek, Vegeta believed he'd achieved the Super Saiyan transformation when facing Frieza, only to be sorely mistaken, resulting in his own temporary death. It's also important to point out that Goku made a similar decision in his own fight with Frieza, allowing the tyrant to reach full power so he could definitively prove that he was stronger than him.
And yet, while Vegeta's move to deliberately step aside as Cell reached his perfect form was one of the most selfish things he's ever done, it is the most telling insight directly into his character. The lengths Vegeta underwent in a failed bid to convince the world he was the strongest fighter, along with his own misplaced overconfidence in himself, is precisely what makes him more endearing to fans than the rather predictable Goku. The fact that it comes from a moment when Vegeta could have potentially saved the day singlehandedly -- and earned the adulation and respect he so desperately craves -- underscores the sequence with a tragic irony and sets the stage for him growing more as a character moving forward.
Vegeta is his own worst enemy in a way, which is made crystal clear by the botched battle against Semi-Perfect Cell. Without this exchange, the Saiyan Prince never reaches the redemptive heights that audiences would later see in Dragon Ball Super and becomes the more heroic figure he is known to be today. And there is something cathartic about seeing the proud, boisterous Vegeta being put in his place and epically proven wrong every now and again, reminding him that he is something of the underdog fans love to root for.