Sanders is living like an ordinary man. Commuting to his job via train, taking out his own garbage, doing his shopping, walking in the streets. Many stories are out in social media from people who regularly encountered him on the streets during his senate stint, in ordinary settings.
Thus, Sanders sees what's going on out on the main street. What issues people are having. What are the problems they are fighting.
He sees what's wrong with US. He sees people's suffering. He feels the pain. That is even before his ~40+ years history as an activist and a politician fighting for the people.
After doing that for 40 years, seeing real suffering and trying to fix it, you become more down to earth, with the weight of the world on your shoulders. A responsible, mature person.
Politicians who are born into privileged class, living secluded and out of reach from the society do not have that 'problem'. Just look at Clinton and how she can easily tell that "1500 deaths in Libya is a minuscule number" in the debate, or claiming to be 'dead broke' while making $200,000+ a speech. Or how she thinks middle class is people $250,000/year.
Ms. Clinton’s wealth has detached her from reality, particularly in regard to her definition of the middle class. With her promise not to raise taxes on households making $250,000 or less a year, she ignores the most recent, which find the median household income is $53,657. Comparing the struggles of a household that makes $53,000 annually to one making nearly $250,000 annually suggests that Ms. Clinton has a complete misunderstanding of what middle class in America actually means.
That's what enables her to snarkily 'laugh off' after being asked when she will release her Goldman Sach speech transcripts, in a cheerful manner.
Someone like Bernie, who has seen how people are struggling because of that very Goldman Sachs and its fellow banks, would not take something like that lightly.