Why can't I always answer the why?

Why is it absolutely important that every student in my class have to know this? This question is always a real struggle for me. I mean, some small percentage of my students will actually work in a field where the content that I teach them will be an actual part of their life. So, why do I teach it. The easy answer is because I have to. It's on the test and it's important for understanding science, but that doesn't suffice to explain to a student why they should care about learning it. I read somewhere the other day, I don't remember where, someone said something to the effect that school should be important because it's where we are presented with problems from all points of life and we learn to solve them through all different means. What do our classes look like when that becomes the focus of every class? What's some topics that you have no clue why your students would really need them to be successful in life? Should we be asking the what do we plan to teach the kids to do through this content? That seems like something more like  I would like to do, but what does that look like?