Bcuz he can retract his teeth!!! Didnt u notice that in the movie?? While Hiccup was giving him some fish and at first he was toothless then his teeth retracted when he got the fish....

Why you can't understand that?!
Of course because of Hiccup! XD

Does this help? Look at the arrows.

At first toothless appears to not have any teeth, but when Hiccup feeds him the fish you see teeth.

At first Hiccup couldn't see his teeth cuz Toothless can retract them. When Hiccup feeds him he shows his teeth to chew food. And in the books he actually doesn't have teeth except for 1 that comes in then gets knocked out. Man, you guys are so dumb sometimes you worry me.

BECAUSE IN THE BOOK HE REALY HAS NO TEETH! god...

Because when he first met him he was toothless. Duh

Riku: This is why. Toothless!

Toothless: *comes over with his teeth showing*

LoveLife: See he has teeth!

Riku: *Does a relistic fake punch as Toothless retracts his teeth* No more teeth.

Aha, u (lovelife324) sound like Dagur in Cast Out p.II when he moans about the name Toothless -.- anyways, it is bcuz when Hiccup first met Toothless, and he was about to feed him the fish, he noticed he didnt have any teeth (or so he thought) and Hiccup says:
"huh. Toothless. I could have sworn u had..." then Toothless extends his teeth out and takes the fish - "TEETH!!!"
And there it is. The whole story behind. XD

Y'all pretty much described it, but im still gonna say:

UM, CUZ HE LOOKED LIKE HE HAD NUU TEETH IN LE MOVIE THEN HE SHOWED EM AFTER HICCUP GAVE HIM THE FISH :I

Hiccup first saw Toothless with retracted teeth so he though he was, well, toothless.

i think it's because he can make his teeth go in his gums so he can close his mouth so he is toothless get it?

When Hiccup and Toothless first met, Toothless didn't show his teeth.