There are obvious advantages to trade, in that both parties gain. Surely America would be much less wealthy if each U.S. state could raise tariffs and other obstacles against products of other states. Treaties lowering trade barriers generally have long-term advantages to all the nations involved. There are short-term costs, however, as relatively less efficient producers in each nation have to adjust, or find other work. Thus it makes sense to find ways to compensate those negatively affected by the deal.
Whether on balance the advantages of TPP are worth the costs is certainly worthy of study and debate. Shouldn't we all do our homework before we endorse or oppose it?