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Oh well the younger Tien had this character traits.
Just be thankful man. ^^

This is just my opinion of course, but Kami should stop underestimating humans, they not only sealed Piccolo quite easily actually, but they proved themselves against his children (Tien especially) time and time again. Still, I guess he only has the Earth's best interests in mind...

I think Kami knows the human race is capable of achieving great things. He has already seen this 300 years ago, when Master Mutaito's students wiped out most of King Piccolo's minion children as well as Mutaito himself sealing Piccolo away.

Kami is distrustful of is the human race, since he has seen mostly selfishness and bloodshed from them in the 300+ years he has been Guardian of the Earth.
All of that changed when he met Goku, a monkey-tailed boy who he knew was not of this world, yet had been raised among the humans as one of their own, and became more pure-hearted than anyone imaginable. And the fact Goku used the Dragon Balls to not only wish his friends back, but also everyone on Earth killed by King Piccolo and his sons.

I honestly think Goku reminded Kami a lot of himself: a child amnesiac, completely unaware of his extraterrestrial origins, yet was raised under the care of humans. While Goku went on to be a happy, pure soul, Kami was spiritually torn apart, and had to physically split that evil from his soul. Perhaps Kami was a little jealous of Goku...

But enough of me rambling. Kami is slightly prejudice of humans for these reasons, and needed more time to assess the validity of the actions of our Earth's heroes in Universe 9. His test was certain the human race could defeat Piccolo, and now he is showing the Z-Warriors how far they still have to go.