There is a beautiful story in the Srimad Bhagavatam about Mahadev, Lord Shiva. Once upon a time when the devas and the asuras were churning the ocean of milk for nectar, massive quantities of poison came out. They were seeking nectar, but sometimes when we want nectar, first we have to remove all the poison.

That’s kind of the way our sadhana is, our spiritual practice. The real nectar of prema, or love – love for God and love for all beings – is within us. Our spiritual practice is just to remove all those things that obscure our connection to that love. So sometimes when we are doing our meditation or our chanting, a lot of poison starts coming out of us – lust, envy, anger, greed, illusion and arrogance – and we actually see all this stuff that is already there. That should not discourage us. We should not give in to those things; rather if we just tolerate them and continue with our spiritual practice, gradually it will all be removed, and then the real nectar which we are all seeking will be forever ours.

So this poison from the churning of the Milk Ocean spread everywhere and the great rishis and devas approached Lord Vishnu, who took them to Shiva. When Shiva saw the plight of all living beings on the verge of extermination, by his mystical power he took massive oceans of this poison and concentrated it so that it could fit in the palm of his hand. We can’t even imagine the power of that concentrate. And because this poison had to go somewhere, he drank it. His throat became very black in colour. This black throat is one of the features of Shiva that is most honoured, and is considered to be the ornament of his compassion. He is known as Neelkantha or Kalkantha. There in the Bhagavatam there is a beautiful verse in relationship to this story:  the service that is most pleasing to Bhagavan is when we make sacrifices without selfishness, out of compassion, for others within this world. – Radhanath Swami

tapyante loka-tāpena
sādhavaḥ prāyaśo janāḥ
paramārādhanaṁ tad dhi
puruṣasyākhilātmanaḥ
 
It is said that great personalities almost always accept voluntary suffering because of the suffering of people in general. This is considered the highest method of worshiping the Supreme Personality of Godhead, who is present in everyone’s heart. Srimad Bhagavatam 8.7.44