There was no good in 'goodbye'. Tadashi was quite sure of it. That's why he always said 'see you!' or 'later!' or sometimes just 'bye!'.
To him, it didn't hold the unspoken promise of seeing each other again after their temporary separation, unlike the others. It didn't imply that he would miss the person until the next time they meet again.
Perhaps, when he said goodbye to his parents that day, neither they nor he broke a promise. (Because promises shouldn't be broken.)
Although they knew how important he was to them, and how much he cherished them, Tadashi would never, never say goodbye to Hiro, or Aunt Cass, or even Mochi.
Although they only knew each other for a year, he was inexplicably attached to his group of peers in San Fransokyo Institute of Technology. He loved spending time with them. He hoped he would never have to say goodbye to them too.
They were the lights of his life.
So when one particular light— one in the form of a girl obsessed with speed and bicycles— grew brighter and brighter and he first held her hand, he didn't want to let go.
When he finally had to, near the end of their date that night, she said, "It was fun. See you later, Hamada."
He grinned and nodded, "See you, GoGo."
(Because they weren't the kind to break promises.)
Then there was a light. It was a column of light that trapped his professor within its burning walls.
He looked at Hiro, who was pleading him not to go. Then he looked at GoGo, who was running toward them.
He wouldn't say goodbye.
He promised them. He promised him. He promised her.
"Would you ever say goodbye to me?" She asked him one night as they watched the stars bright in the sky.
He gave her a smile. "Never."
Perhaps he should have, Tadashi thought to himself as he felt his beloved cap fly away from his head.
Perhaps he should have said goodbye.
(Because he never liked breaking promises.)