Yoni's motivation in talking to Sergei is to get good material for the documentary he is making. This compares to the goldfish motivation because the goldfish is motivated to fill someone's life with good things.

The theme revealed through the way Sergei acts is the theme that happiness is never eternal and there will always be times when it won't be there.

Although you did not present the text to which this question refers, we can see from the context of the question that you are referring to "What of this Goldfish, Would You Wish?"

By reading this short story, we can say that:

The story shows how a poor boy named Yoni has the brilliant idea of filming a documentary to sell to a large television network.
The documentary consists of filming people who will answer the question "What would you want if you had a magical fish that could grant wishes?"
While interviewing people, Yoni ends up arriving at Sergei's house, which does have a magical wish-granting goldfish.
Sergei is a foreigner and is in a moment where he considers himself happy.
This is because, in addition to having a magical fish, he is living the way he wanted.
However, he doesn't want to reveal that he has a magic fish to Yoni and Yoni's insistence on interviewing him causes his happiness to slowly dissipate.