Why did drona help the kauravas

Question

Since Drona is the teacher of both the Kauravas and the Pandavas, why did Drona help the Kauravas by fighting with the Kauravas?

In stories, it shows that he is fighting on the Kauravas side. Why?

Thanks in advance!

Answer 1

Droṇa himself gives the reason fighting for the Kauravas when right before the war, Yudhiṣṭhira seeks his permission to fight his own guru:

Yudhishthira said,

I ask thee, O invincible one, how I may fight without incurring sin, and how, with thy permission, O regenerate one, I may vanquish all my foes?

Drona said,

If, having resolved to fight, thou hadst not come to me (thus), I would have cursed thee. O king, for thy complete overthrow. I am, however, gratified, O Yudhishthira, and honored by thee, O sinless one. I permit thee, fight and obtain victory. I will also fulfill thy wish. Say what thou hast to say. Under these circumstances, battle excepted, what dost thou wish? A man is the slave of wealth, but wealth is not one's slave. This is quite true, O king! Bound I have been with (their) wealth by the Kauravas! It is for this that like a eunuch I shall fight for the sake of the Kauravas. It is for this that like a eunuch I am uttering these words--Battle excepted, what dost thou wish? I shall fight for the sake of the Kauravas, but will pray for thy victory.'

Yudhishthira said,

Pray for my victory, O regenerate one, and counsel what is for my good. Fight, however, for the Kauravas. This is the boon solicited by me.

Drona said,

Victory, O king, is certain for thee that hast Hari for thy counsellor. I (also) grant thee that thou wilt vanquish thy foes in battle. Thither where righteousness is, thither is Krishna, and thither where Krishna is, thither is victory. Go, fight, O son of Kunti! Ask me, what shall I say unto thee?

Yudhishthira said,

I ask thee, O foremost of regenerate ones, listen to what I have to say. How shall we in battle vanquish thee that art invincible?

Drona said,

As long as I will fight, so long victory can never be thine. (Therefore) O king, seek with thy brothers, for my speedy slaughter.

Yudhishthira said,

Alas, for this, O thou of mighty arms, tell (us) the means of thy death. O preceptor, prostrating myself I ask thee this. (My) salutations to thee.

Drona said,

The foe, O sire, I see not who may slay me while standing in battle I am engaged in fight, with wrath excited, and scattering (my) arrowy showers continually. Except when addrest for death O king, having abandoned my arms and withdrawn (in Yoga meditation) from surrounding sights, none will be able to slay me. This that I tell thee is true. I also tell thee truly that I will cast off my arms in battle, having heard something very disagreeable from some one of credible speech.

Answer 2

Drona and Drupad were chums though poles apart by wealth and status. Drupad intentionally or by ignorance promised him a bounty when he ascends the throne. He feigned ignorance when Drona came to his palace and asked for it.

Drona cursed him and warned him that a day will come when he will be strong enough to fight him.

Drupad was Pandavas' brother in law by the virtue of Draupadi marrying the five brothers.

When time came to choose the sides, Drona wanted to teach Drupad a lesson and hence joined Kauravas.