why did montmorency was excited to go to the trip?

Montmorency wasn't excited. In fact, he was opposed.

The only one who was not struck with the suggestion was Montmorency. He never did care for the river, did Montmorency.

“It’s all very well for you fellows,” he says; “you like it, but I don’t. There’s nothing for me to do. Scenery is not in my line, and I don’t smoke. If I see a rat, you won’t stop; and if I go to sleep, you get fooling about with the boat, and slop me overboard. If you ask me, I call the whole thing bally foolishness.”