It's rude to not accept a gift from someone.
Sasha is celebrating her birthday and somebody brought her a gift she didn't ask for.
Sasha wants to enjoy her birthday.
Sasha appreciates the thought that went into the gift and thanks the guest for it.
Since Sasha didn't ask for the gift she tells the person to just keep it.
The guest feels good about themselves and enjoys the party.
The guest is offended that Sasha didn't accept the gift and leaves the party.