I think veto's should only be used if there is apparent collusion, or a vet is exploiting a new owner. If both owners are trying, in good faith, to improve their teams then it shouldn't be a problem. Of course, owners have the right to veto for any reason they want, but i probably wouldn't stick around a league like that for long.
By the way, did you explain your trade on world chat before the trade was veto'd?