因为介词against有“靠，倚”的意思。如：I put the ladder against the wall.除此之外，还表示“与…竞争，对垒”。如：They played against the best team and won the game. 还有“反对”的意思。如： I'm against the plan though many people are for it.还有“预防，防止”的意思。如：Have you had an injection against the disease?