Happy are the warriors, oh Pārtha, who find such battle opened as a door to heaven, presented of its own accord

The battle is upapannam (presented, arisen) yadṛcchayā (by favorable chance, by good fortune). It does not have to be sought; it comes to one who has duty.

Svarga-dvāram apāvṛtam (door to heaven open) — death in battle with duty fulfilled guarantees svarga (heaven). This is the traditional kṣatriya-dharma: sacrifice of life for dharma.

Sukhinaḥ (happy, fortunate) — those who have this opportunity. It is not tragedy but privilege. Arjuna should be grateful, not lamenting.