The Queen and Zara's joint love of horses and horse riding also play a part.
“They have a lot in common as they talk horses and the Queen has invested in several horses for her,” Ingrid revealed.
The Queen's granddaughter and her husband like to keep themselves separate from ”royal dramas”, something Elizabeth II respects.
“They are certainly light relief. She can be herself around them and she does not care for the alleged feuding among William and Harry," Ingrid told the publication.
Zara, 38, and her brother Peter, 41, are the Queen's first grandchildren, so she's always had a special bond with Zara.
Ingrid also said the Queen prefers the company of “ordinary people like Mike more than she does those that pander to her”.
Ingrid added: “That is why she gets on so well with her racehorse trainers.
“Mike is a down-to-earth Yorkshireman who makes her laugh and makes Zara very happy.
“I’m not sure if she has a similar sense of humour to Mike, but she does have a sense of the ridiculous which he seems to have too!” she told the publication.