Yamato and Taichi would have nicknames for each other, rarely use them especially Yamato.

I think whenever Yamato would use a sweet nickname. In normal sense, it would endearing. But if Yamato does it, it usually means Taichi is in trouble.

Yamato used the sweet nicknames when he pissed off at Taichi.

Taichi knows it and would kind of be cautious.

Taichi would use them as a well to calm Yamato down when he's in one of his moods or as comforting mechanism