Slack CEO Stewart Butterfield. Slack

One of the first things Slack CEO Stewart Butterfield says to job candidates during interviews is, "Tell me your story."

He wants to know: How much of your success came down to luck, and how well do you appreciate that factor?

Butterfield attributes 98 percent of his own success to luck, and prefers candidates who cite a similarly high percentage in their answers. He says he has known people who weren't professionally successful despite their talents, character, and other merits.

"It's difficult for me to think that there was something wrong with them," he said Monday during the Wisdom 2.0 conference in San Francisco.

Hiring people who realize hard work doesn't pay off without a few big breaks isn't just about cohesion in the workplace or steering away from an overly cocky crowd in the office. Butterfield says the humility he's seeking in candidates indicates empathy, and Slack needs its employees to be empathetic for the company — which has proclaimed itself the fastest growing business app ever — to succeed.

"It's very difficult to design something for someone if you have no empathy," he said.

Valued at $2.8 billion, Slack grew to 1.7 million users within 20 months of its launch two years ago. The company now has about 380 employees.

Butterfield likens the role of empathy in creating and scaling something customers like to hosting a guest in your home. An empathetic host anticipates a person's needs, taking care of small things like placing towels on the edge of a bed so there's no confusion as to which towels the guest should use.

Expanding on the topic of anticipating the needs of customers, Butterfield mentioned that diversity among employees is key. If you don't have people who come from different backgrounds and experiences, you'll miss out on meeting the needs of groups of customers.

"If you're not hiring from some groups of the population, then you're obviously missing out" on something, he said.