Veto can be used if the trade is very lopsided in the view of other owners. Sometimes, an owner involved in a trade is not able make an accurate assessment of the market value of his players, and enters into a bad trade. It happens. That's why other owners have the ability to veto if they assess (on a partial majority basis) that a trade is unfair or not, regardless if the owners involved in the trade "think" that they are entering a "fair" exchange. Simply put, owner/s involved in a trade themselves failed to properly assess the market value of the players involved in the trade. Two owners cannot force other owners not to veto; if so many other owners vote veto, then there is no choice but to cancel the trade (and it is difficult to obtain a number of veto votes unless a trade is really perceived by other owners as unfair/vetoable).
However, if the trade is just a bit in the advantage of one owner, but happened because of reasons such as "need other positions/stats" (and not a pure upgrade on the same position filling the same categories), then there will be less or no reasons for other owners to veto. Even if 1 or 2 owners veto for selfish reasons, if the number of vetoes doesnt reach the needed number, the trade can push through.