Well, Sokka isn't all that trusting, so there has to be something about this guy that's good. Is he like Zuko? You know, back in his 'burn it with fire, capture the Avatar' days?

[Honestly, if the guy's that unstable and untrustworthy, Toph doesn't want him around. Bad enough watching her back and her friends' backs, but watching them for enemies AND watching for someone on there team to betray them? Too much of a distraction.]