"I affect many things, and can inflict my effect on anyone, at anytime. I have different forms, and they have different effects on people. Humans have built defences against some of my forms, but not all. I can be useful, depending on the situation. What am I?"
Not love nor water...i was too thinking it might be some sort of weapon but it doesnt really fit with the clue, I was 100% sure it was weather or related