It cuts the other way, too. If someone tells me, "I must have a diamond" then I know that person values her friends' thoughts or her appearance to strangers over her own rational thoughts. Not interested. (Alternatively, she has no rational thoughts. Also bad.)
It sounds quaint and old-fashioned, but a man should give everything to a woman he hopes to marry, and demand nothing in return, without worrying about whether or not it's sensible. It doesn't make sense until you realize that she only wants to do the same for you. She will commit to unconditional satisfaction of your wants.
This makes life seem even more meaningless than I usually think it is. One person does whatever he thinks the other person wants, in exchange for that in return. A silent existence devoid of interaction, sacrificing your happiness so that the other person is happy to sacrifice their happiness.
All that's left after this is reproduction. Is that all human relationships are about?