Why did Tenshinhan decide to leave forever?

[ahill1]

When everything with Cell was said and done, he said they wouldn't see each other again...

Why, would you say? Maybe he doesn't consider them "friends", but more so "battle allies"?

I felt sad when I saw that for the first time, actually lol

 

[Captain Cadaver]

It was honestly really forced, just being Toriyama's way of saying Tenshinhan wouldn't matter anymore and trying to phase him out of the plot (which obviously didn't stick). The reasoning provided for him not being with the group in the Boo Arc of his Ki being too concealed was a far better way of removing him from the plot, not to mention his comments in the Cell Arc of the gap between he and the Saiyans being too great already served the purpose of showing his fading relevancy.

For an in-universe explanation though, perhaps this fading relevancy and constantly having to keep comparing himself to Goku and co. is what convinced him it was better to train to improve himself rather than in surpassing them and the goal he was constantly trying to surpass (Goku) was gone now. It's not as if he's that social either, considering he didn't show up for the Kame House reunion or anything that didn't involve battle.

 

[Future Warrior]

Because his main driving force (Goku) is now gone. Since it's assumed he's never getting brought back to life, he has no reason to tag along with the group unless some bigger threat shows up that requires him.

 

[SSJ2]

He decided to return to his home planet of the 3 eyed people where he could rightfully take his place as the strongest bald meng.

 

[ahill1]

Ten only sees them as battle allies, rather than friends, then, right? I think his only friend may be Chaozu.