The Beast is a very complex character.
He goes through so many changes in his life and to me they are very belivable and here I will discuss why.
First his childhood:
Not much is know of his life before the enchantment except that he was like the prolouge says "Spoiled, Selfish and unkind" but that is all I need, I belive that he got the position of master at an early age and that made him that way.
I quess his parents died sometime before the story and that gave him too much power for a child and it gave him the character flaws that the Enchantress cursed him for.
Next his Teen years:
His teen years should have been hard enought as a human but being turned into a Beast could only have made it worse, and his transformation didn´t only alter his appearence but his view on life as well.
As the years passed he became more and more feral and had Belle never come to the castle he would have stopped wearing clothes and would also had lost the ability to speak and think as a human.
His anger at the beginning of the movie is normal teenage anger, confusion and awkwardness combined with his own guilt over what he did to himself and his servents so long ago.
When Beast have turned 20 he is an angry and shy person and he have a big problem with controlling his temper.
When Belle later comes to the Castle he sees his only chanse to be human again and tries to change but the early attemps is fruitless.
Later on after he saves Belle´s life he learns how good it feels to be kind and he succeds with changing for her even if it goes slowly.
Proof of his changing personallity is first of all he gets less angry, he walks more like a human and begins to wear clothes, When he first appears he wears a cape and ragged trousers but as his relationship with Belle develops he starts to take care of his appearence more by wearing more refined clothes like a shirt and whole pants.
Another example of him changing is his initial refusal to fight Gaston on the roof, His love for Belle "tamed" him and made him more calm even if he was depressed during the fight.
The Spell is broken and Beast returns to being the Prince, His love for Belle broke through his despair and anger and turned him into a kind and compationate person.
I love his spiritual transformation because it shows that love can really perform miracles and that makes me belive in the real power of love.
I hope you have enjoyed this article ^^