Śāntideva said this quote

For a practitioner of love and compassion, an enemy is one of the most important teachers. Without an enemy you cannot practice tolerance, and without tolerance you cannon build a sound basis of compassion. So in order to practice compassion, you should have an enemy.When you face your enemy who is going to hurt you, that is the real time to practice tolerance. Therefore, an enemy is the cause of the practice of tolerance; tolerance is the effect or result of an enemy. So those are cause and effect. As is said, Once something has the relationship of arising from that thing, one cannot consider that thing from which it arises as a harmer; rather it assists the production of the effect.

~ Śāntideva

What need is there to say more?The childish work for their own benefit,The Buddhas work for the benefit of others.Just look at the difference between them.If I do not exchange my happiness, for the suffering of others, I shall not attain the state of Buddhahood.And even in Samsara I shall have no real joy. The source of all misery in the world lies in thinking of oneself;The source of all happiness lies in thinking of others.

~ Śāntideva

When happiness is liked by me and others equally, what is so special about me that I should strive after happiness only for myself?

~ Śāntideva

When fear and suffering are disliked by me and others equally what is so special about me that I protect myself and not the other?

~ Śāntideva