You don't sound like a very romantic person. There are other things in the world more expensive than diamonds, but she didn't ask him to buy her a skyscraper or Renaissance painting. Different gifts have different meanings, and it's no less logical than different words having different meanings. People associate specific meanings with specific types of stones, flowers, beverages, you name it. You can't substitute "I greatly enjoy the time we spend together" for "I love you."
I must admit to some confusion. I am closely rereading my original comment, and I am failing to find where I suggested that buying a skyscraper is the same as buying a ring.
