When we use words to say how we feel, the more relevant concepts and distinctions that we know, the more precisely we can express our feelings. So you might think that the number of relevant distinctions we can express on a topic rises with a topic’s importance. That is, the more we care about something, the more distinctions we can make about it.
But consider the two cases of food and love/sex (which I’m lumping together here). It seems to me that while these topics are of comparable importance, we have a lot more ways to clearly express distinctions on foods than on love/sex. So when people want to express feelings on love/sex, they often retreat to awkward analogies and suggestive poetry. Two different categories of explanations stand out here:
1) Love/sex is low dimensional. While we care a lot about love/sex, there are only a few things we care about. Consider money as an analogy. While money is important, and finance experts know a great many distinctions, for most people the key relevant distinction is usually more vs. less money; the rest is detail. Similarly, evolution theory suggests that only a small number of dimensions about love/sex matter much to us.
2) Clear love/sex talk looks bad. Love/sex are to supposed to have lots of non-verbal talk, so a verbal focus can detract from that. We have a norm that love/sex is to be personal and private, a norm you might seem to violate via comfortable impersonal talk that could easily be understood if quoted. And if you only talk in private, you learn fewer words, and need them less. Also, a precise vocabulary used clearly could make it seem like what you wanted from love/sex was fungible – you aren’t so much attached to particular people as to the bundle of features they provide. Precise talk could make it easier for us to consciously know what we want when, which makes it harder to self-deceive about what we want. And having available more precise words about our love/sex relations could force us to acknowledge smaller changes in relation status — if “love” is all there is, you can keep “loving” someone even as many things change.
It seems to me that both kinds of things must be going on. Even when we care greatly about a topic, we may not care about many dimensions, and we may be better off not being able to express ourselves clearly.