why do the chinese people celebrate dragons?

You may refer to the Dragon Boat Festival, don't you? If so, it is not celebrated for dragon, but to memorate a patriot - Qu Yuan, who was a minister in the State of Chu in Chinese history! Faced with the perishing of his state, he couldn't bear that and didn't wanna surrender so drowning himself. The fishermen nearby found that to take their boats to find the body of Qu Yuan. Many boats ran together to avoid his body was eaten by fish. The Dragon Boat Race was inherited from this!

Chinese refer to dragon (Long, to be more accurate), as their ancesters. Actually, we call ourselves the descendants of Long. In traditional Chinese culture, Long is a deific existence, bringing us rain and good harvest. So basically it is quite different from the image of dragon in traditional western culture.