The Avalanche has a problem with authority and hates being told what to do. Denver was originally a frontier town populated by gamblers and miners and as a result, the Avalanche has a deep-rooted gambler mentality. He's willing to take chances that most of the other Guardians wouldn't dare, not because he actually believes that he can accomplish the act in question, but simply because he wants to see if it's possible.
The Avalanche is frozen, more specifically; he's a giant piece of walking frozen rocky earth. He takes this form as a direct result of Denver's location at the base of the Rocky Mountains. To keep him frozen, the Avalanche has a cryogenic titanium suit and has the ability to freeze anything he touches. He has the super ability of cryokinesis, which allows him to control the frozen environment around him and he has the ability to induce an actual avalanche whenever he chooses, provided of course that conditions allow for it.