Arjuna’s personal call-of-character (Swadharma) is that of a leader of his generation (Kshatriya) and as such, when his generation is called upon to answer a challenge of an organised un-Aryan force (Adharma), it is his duty not to waver but to fight and defend his sacred national culture.

To the leaders of people, there can be nothing nobler than to get a glorious chance to fight for a righteous cause. Here Arjuna has been called upon to fight a righteous war wherein his enemies are the true aggressors. Therefore, it is said that such a chance comes, indeed, only to a lucky few. That a king must fight on such an occasion is vividly brought out in the Mahabharata.

AND REGARDING OTHER REASONS WHY THE BATTLE SHOULD BE FOUGHT, THE LORD SAYS:

Api, even; aveksya, considering; svadharmam, your own duty, the duty of a Ksatriya, viz battle — considering even that — ; na arhasi, you ought not; vikampitum, to waver, to deviate from the natural duty of the Ksatriya, i.e. from what is natural to yourself. And hi, since that battle is not devoid of righteousness, (but) is supremely righteous — it being conducive to virtue and meant for protection of subjects through conquest of the earth –; therefore, na vidyate, there is nothing; anyat, else; sreyah, better; ksatriyasya, for a ksatriya; than that dharmyat, righteous; yuddhat, battle.