Gustav probably never talked about his mother, David realized. And I didn't ask. I think it was Ursula who said that Gustav's mother died when he was young.
Until now, there had been no opportunity to ask him about it, and anyway, David thought the question might be upsetting to his boyfriend. If people don't talk about something, it means that either the thing doesn't matter to them, or it hides something painful behind it. Something or someone. David sincerely doubted that his mother was indifferent or unimportant to Gustav. So he suspected that there might have been something unpleasant for Gustav with the memories of her.
She died when he was young. It was certainly nothing pleasant.
Yet now Gustav wanted to take him to a place that was associated with his mother. There was trust in this declaration and a desire to share himself precisely with David. He could not look for excuses or refuse.