Even though he is evil at heart, Vegeta is  perhaps one of the most strong-willed characters in the series. Like  Son Goku, he is extremely industrious, constantly striving to be  better, but different purposes drove Vegeta's spirit. Goku fought to  defend the lives and others and to challenge his limitations while  Vegeta fought to satiate his bloodthirsty appetite and die hard  pride, which is unwilling to accept defeat. This gradually changes  throughout the series.

The primary driver of his will is Son  Goku. Vegeta's defeat in his first encounter with Goku and his  unwillingness to accept Goku being more powerful had constantly  fuelled his strong will to continue to train himself. After seeing  Goku turn into Super Saiyajin and defeating Freezer, he grew even  more jealous of Goku achievements and immersed himself in intensive  training. After much sweat and frustration, he finally became Super  Saiyajin. He only enjoyed a fleeting moment of pride in destroying  #20, before being defeated by #18.

Vegeta first catharsis  occurred during Son Gohan battle with Cell. Because of his rash  decision to attack Cell when he did not have the power to, had  caused Gohan arm to be seriously injured when Gohan saved his life.  Not only was he not even close to Cell ability, but also served as a  hindrance to Gohan. That was the first time Vegeta accepted his  incapabilities by making an apology to Gohan.

By the time  Dämon Boo appeared, what used to be a cold-hearted Vegeta changed.  Right before he proceeded to sacrifice his life in an attempt to  destroy Boo, he told Trunks that he cared about him and how he felt  bad about giving Trunks much less attention then Trunks  deserved.

During Son Goku's final battle with Dämon Boo,  Vegeta goes through a complete catharsis. He realized that Goku's  pure and kind heart gave Goku the power that Vegeta never had. He  finally realizes that the good ultimately prevails.