As long as he doesn't see the reflection of his qualities in them, he will continue to cleanse them, even if it hurts him too.

A beautiful mythological story about Lord Shiva Retold and shared by Sangeeta Bodhi Das:
One day Parvati got very angry with her husband, & marched up to his abode.... "Why are you giving so much pain to your devotees? They love you so much & you put them through such tough tests. I am a mother, I can't see their pain. They call you BABA, & you push them so much."

Disturbed in his meditation, Mahadev, opened his eyes and smiled at his wife. He sent her to a man on earth, "Go to this man & see how he works. You'll get your answer."... and the God of few words, resumes his meditation.

Parvati came to earth & looked up this man, a goldsmith. The man was busy heating some precious metals. He was sitting very close to the fire and had his hand almost inside the flames, heating gold and silver.
"Why are you burning this?", she asked.
"I am not burning it, I am just getting rid of all the impurities in it."
"Are you going to sit like this whole day, putting your hand in the fire?"

"Yes", he smiled, "Not only will I keep doing it, also I cannot take my eyes off it even for a mili-second. A little carelessness & I will lose something very precious to me. I will have to keep watching it.”
“But how will you know that it is purified?”... she wondered.
“When it starts glowing and shining from within, I will be able to see my face reflected in it.”

Parvati found her answer. She knew that day, that her husband who has three eyes, which are closed most of the times, never takes his eyes off his precious devotees, humanity. As long as he doesn't see the reflection of his qualities in them, he will continue to cleanse them, even if it hurts him too.