And/or that you need to demonstrate your feelings for someone else through an object that has inflated worth...
The desire to control your partner that is displayed by many people in these diamond threads is a really unpleasant character flaw.
It's not about "control", it's about human relationships and value.
Isn't that a good reason to steer clear of anybody who insists on being bought diamonds?
Edit: Unless you like buying diamonds, or have so much money it doesn't matter.