When I first started teaching, I hated the question, "why are we learning this?" I felt that it was a challenge to the way I was doing things, my passion for the content I was teaching, and took it as a personal attack.
I would like to challenge you moving forward to embrace the "why". A student asking why simply means one of two things. Either they are disengaged in the content and are displaying avoidance behavior, or they are analyzing the content and thinking deeper about it's purpose.
If the first of these two are true, students are avoiding, we need to be reflective and think about engagement. Some questions to ask yourself might be, do you need to build in more movement or purpose to the learning? Is the content relevant to students lives? Can you build that connection? Is the content truly something you should be spending time teaching?
The second reason is that students are thinking about the value of the content. They are reflecting on the relevance of the content in an ever changing world. It is alright, do not take it as a challenge to your hard work, rather, embrace it. Take time as a class to build relevance and reflect. Maybe the time spent on the content really should be minimized.
The final suggestion I would like to leave you with is for those of you that truly want to embrace the "why". All good lesson planning starts with looking at a standard and forming an objective or essential question. This should be shared with your students so they know what they are expected to learn during the lesson, but also as a formative check at the end of the lesson. What if you started asking yourself why are you teaching this to your students and started teaching your content with a specific purpose in mind? My challenge to you is to listen to your students. At the end of your lesson, ask your students why they think they are learning this. See if their "why" aligns to your "why". That's where the learning happens. Continue to be reflective and let your students help you become a better teacher.
Happy Teaching!