Bhagavad Gita

ಯದೃಚ್ಛಯಾ ಚೋಪಪನ್ನಂ ಸ್ವರ್ಗದ್ವಾರಮಪಾವೃತಮ್ ।
ಸುಖಿನಃ ಕ್ಷತ್ರಿಯಾಃ ಪಾರ್ಥ ಲಭಂತೇ ಯುದ್ಧಮೀದೃಶಮ್ ॥೩೨॥
yadṛcchayā copapannaṃ svargadvāramapāvṛtam |
sukhinaḥ kṣatriyāḥ pārtha labhaṃte yuddhamīdṛśam ||32||

Gist of the sloka:
O Partha, such wars can only happen due to the will of Lord. Fortunate are few who get opportunity to fight in such war. If you fight such a war [and die], the doors of the heaven is open for you. If you win, then the lordship of the kingdom here on the earth and post death heavenly position is yours.
To ignite a war for no reason is wrong. In this scenario, the war was imposed on the Pandavas. The war is happening because the Lord willed it that way. Nothing moves or happens in this world independently of Lord. One should therefore accept the things/events as it happens with this belief.
For those who fight against evil, the doors of the heaven are opened for them. The chance to fight in this war is a unique opportunity which rarely happens to a kshatriya. It must be due to their accumulated ‘punya’ [meritorious deeds pleasing the Lord]. It is inappropriate for a kshatriya to sit idle during such opportunities.
This clears up any doubts one has, as to why Lord Krishna is insisting on the war. All other dharmas need to be in alignment with Sanathana dharma. One should therefore follow the Sanathana dharma and act accordingly.
The Sanathana dharma indicates one should fight and stand up against evil forces. One should therefore perform their given duties and do the needful. Here Arjuna being a kshatriya should fight against the forces of evil without considering his relationships or companionship.