A SIP Day Question

Today we had a half day with students and then meetings all afternoon. I pose a simple question...

Why is it that when students write us a note or say something nice we feel like we matter, but when a student is hurtful we're not supposed to let it bother us because it's "meaningless"?

Just a thought as I watched the video clip that our principal showed us today during our meeting to help to inspire good teaching.