The Dragon Ball Super anime introduced the concept of a multiverse to the Dragon Ball franchise, and presiding over all the universes is the omnipotent being Grand Zeno. Bearing the title of the Omni King, Zeno has the power to erase entire timelines and universes with little more than a wave of his hand. He is feared and respected by all the Gods of Destruction and Supreme Kais, lest they incur his wrath.
However, while Zeno is treated with utmost reverence befitting his sheer level of cosmic power, the Omni King has formed an unlikely friendship with Goku, making the Super Saiyan closer to Zeno than any other character in the franchise. What exactly makes these two such a friendly duo?
Grand Zeno was introduced after the conclusion of the Tower of Destruction, a martial arts competition between warriors from Universes 6 and 7 with Goku's team emerging triumphant for the latter. While initially annoyed that he hadn't been invited to observe the contest of champions, Zeno was thoroughly entertained by the tournament's final match and befriended Goku -- despite the Gods of Destruction pointing out a mortal is unworthy of addressing the Omni King directly. Goku's burgeoning friendship with Zeno would inform the latter half of Dragon Ball Super, culminating in the Omni King overseeing the Tournament of Power, a martial arts battle royale between all the universes that served as the climactic conflict of the anime.
The key to Goku and Zeno's friendship is precisely how naive or unconcerned Goku is by the immensity of Zeno's power and the sanctimony of his title as the Omni King. Even Zeno's attendants, who never leave their charge's side, are visibly afraid of crossing him, worried they may be erased from existence for even the smallest transgression. Goku, however, talks to Zeno as he would any of his friends and equals, even going so far as to give him the nickname "Zenny," which Zeno appears to appreciate. Moreover, Goku is unafraid to make actual physical contact with Zeno, shaking his hand and picking him up which Zeno finds fun, especially in contrast to everyone else walking on pins and needles around him.
The other possible reason for Grand Zeno's friendship with Goku is that both of them possess a childlike sense of innocence. Zeno appears to be primarily motivated by the pursuit of fun and makes Goku promise to keep a playdate with him. Intuitively aware that Zeno is looking for a friend to meet him at his own level, Goku retrieves the Grand Zeno from Future Trunks' alternate timeline and matches the two Omni Kings together to their visible delight.
Grand Zeno is a living example of how much loneliness can come with supreme power. Goku is the one individual in the entire Dragon Ball multiverse capable of cutting through all that to give the Omni King the friend he sorely needs. He's one of the friendliest, most welcoming figures in the Dragon Ball franchise, making friends everywhere he goes worldwide and now into the wider cosmos. This remains true with the Omni Kings, striking up a friendship that helps inform the course of Dragon Ball Super as it extends to multiple universes.
