I think, there is no need to try to dictate how the game goes. If you can play reactive, scout him out really well and know which tech path he chooses, then thats the best thing in the world. Because then you can counter him perfectly and kill him easily. It just sets you in the better spot. It's not better, if you want to change positions and force your opponent into doing something. except when you do something like build 1 or 2 colossi to force him into building vikings and then going ht or something.
but it's also you who wants to play reactive, because then you can find the best composition to kill your opponent, find the perfect expanding times, etc.
you don't necessarily need to play defensively when you play reactionary. You can always do something like: "Hey, he expanded early, therefore he doesn't have many units, therefore I could attack" things, or do other things, like many small engagements, zealot harassment, etc etc. I would even say you can be more aggressive when you play reactionary, because you will always have the better composition against his army and can therefore be more cost effective
that's just my thought about it