What evolutionary purpose does this have?

Why is it, when my son, who completed fifth grade this year, walked across the stage and took his certificate of completion from his teacher, that I teared up like I was going to cry forever?  Why did I feel an ache and sadness while watching the slide show of pictures of the whole school of fifth graders with pictures of my son at various stages in his elementary school career, including the one where he and his sister pose in front of the school sign on her first day of kindergarten?  Why does my throat lump up as the principal and teachers tell the kids how great they've been and how great they'll all do in middle school?

Shouldn't I be so happy that he's growing into a responsible young man and is ready for his next level of development?  That he's going to take his challenges and manage them well?  What makes me so sad and weepy?

At this rate, I am going to need a box of tissues and permission to bawl my eyes out when he graduates from high school, and when he gets married, I may need some people holding me up.

PS--I don't remember such a fuss being made of me finishing sixth grade (which was the top grade in elementary back when I was a kid).  Maybe it didn't impact me all that much.  We'll have to check with my mom and see if she was weepy.