Thoughts on tears

E: Sometimes when I laugh really hard I get tears in my eyes.
M: That's normal. It happens to everyone.
E: OK. Good. Sometimes it also happens when I cough really hard or don't want to swallow my food.
M: That's normal too. Tears happen for lots of reasons. When you're hurt, when you're sad, when you feel sick or tired... Some people even cry when they're really, really happy.
E: That's never happened to me.
M: Someday it will. When you're just so happy you don't know what to do, your eyes start to cry.
E: I bet that would happen if you got me a puppy.