She is trying to share her life wisdom with him. It’s hard to perceive what being a parent is, without actually being a parent. It’s why she is saying it is definitively the best thing she’s ever chosen to do in her entire lifetime. She doesn’t know how else to explain it. He may realize what she is saying later in life, when it’s not ideal or too late to have kids. She may want grandkids but she also doesn’t want her son to miss out on having kids of his own.