May I know the meaning of the sentence "A gentleman is someone who does not what he wants to do but what he should do"
The sentence means, roughly, that there are obligations incumbent on someone who aspires to the status of gentleman (or higher). Cf. noblesse oblige.
Noblesse oblige (/noʊˌblɛs əˈbliːʒ/; French: [nɔblɛs ɔbliʒ]) is a French expression used in English. It translates as "nobility obliges" and denotes the concept that nobility extends beyond mere entitlements and requires the person who holds such a status to fulfill social responsibilities. For example, a primary obligation of a nobleman could include generosity towards those around him.