He met Elizabeth before the war. So if he had intentions to marry her, serving actively in the war was a obvious step to win the approval of both Elizabeth and King George VI.
Plus his family wished to ingratiate themselves as loyal English
He had family members that had other sympathies
Highly unusual family he came from
well, he had to fight if he was still in the UK - he wasn’t a naturalized subject of the UK then in 1941 (he became so in 1947), but he was expected to fight.