To say for instance that something is good or too expensive FOR us, we use 对 /duì/
对 /duì/ is a preposition of a confrontation with some phenomenon
When you want to say that something is too expensive FOR you or that smoking isn’t good FOR health, you use 对 /duì/.
对 /duì/ if FOR – but not in the sense that we have bought something for someone, but in the situations when we are describing attitude TO something or influence of one thing TO another.
Basically we take a phenomenon and put it OPPOSITE us (or someone or something else), we CONFRONT ourselves with it and say what the result of that confrontation is.