The saddest people ...

Nah, did you see his face when Lily was kidnapped? The only time I've seen him unhappy in the whole game

I've never seen Hau sad....

Hau has been doing nothing but smiling the entire game.
Aw, damn, my feels.

That is a little creepy no offense

That looks kinda creepy

*takes litten*

*gives him rowlet*

now we're equal

Well, let's see...
Like Gladion said, Hau's been living in the shadow of the kahuna, his grandfather for his whole life. Yes his grandfather is very good to him as is everyone else, including his mother, but he still has a lot of pressure on his shoulders. He's expected to be just as powerful and wise as his grandpa so he will become Kahuna.
As a young child, probably around 3 or 4, he saw his grandfather go ballistic and burst into tears. Whether it was anger toward Hau or someone/something else, it must be pretty traumatizing to see the one you must look up to be like that.
Of course that followed with Hala becoming very soft towards him, but still.
Next, as soon as the player arrives, Hau already have gotten his starter either as an egg, wild Pokemon or Starter Pokemon, it turns out you've picked the stronger Pokemon and you end up mopping the floor with him.
This continues, with Hala showing more and more interest in you then his own grandson(Blue feels sympathy), and so does the girl he has a crush on and another person he looks up to.
Eventually Hau ends up failing to protect Lillie, with her being the one who stood up and did something, even without a Pokemon. With Gladion continuing to mostly call him out for his facade and weak training, Hau decidest that he must toughen up and find genuine happiness, aside from Malasadas.
He eventually becomes strong and has bonded with his Pokemon, ready to take on the Pokemon league. He gains the respect of Sasuke, Pr. Kukui and all the Trial Captains and Kahunas, including his grandfather, and Lillie.
Still, all of their main focus goes to the main character, who he still can't beat.
And once the main story is over, guess who leaves and seems to have taken up romantic interest in the main character?
Lillie, the girl Hau has a crush on.
Still, throughout almost every part on this adventure, Hau remains cheerful, even when times seem sad. All the attention, even his own grandfather's, has gone to you.
Don't get me wrong, Hau is genuinely happy several times throughout the story.
The reason Hau seems to enjoy battling you and talking to you so much is not only for friendship, but to maybe get attention and respect from you.
He's trying to be helpful.
But in the end, Hau does manage to accomplish a few things.
Respect, strength and knowledge.
But the people closest to him still seem to care about you more.
Well, atleast he has a new friend, and decides that maybe he doesn't need attention to be genuinely happy.
Meh, that's just my theory.
My Game Theory.

I think it would work better with Gladion

But why is he sad? I'm only on akala island in that game. Did something happen to him at some point?

Hau is da best trainer

It's ok Hau! You're still a great Trainer and an amazing friend in my eyes. :<

Sometimes the Jester is the saddest man in the room.