When it is unwanted, usually, or is embarrassing or perhaps a little creepy.
Love letters are a little old-fashioned, these days. Emails, texts, or even some kind of photos are exchanged or sent in modern times (I hear). But when a love letter is calm, moderate and not remotely threatening, why should it inspire anger and even rage? If it is written with sincere sentiments, even if unwanted, it may even inspire pity, understanding or even sympathy.
So why are some people - online, elsewhere - so angry about the "Let's Stay Together" letter signed by 200 figures of various levels of public profile? If you have missed it, it is a letter which, I think, it rather mild in its contents. It says: "We want to let you know how very much we value our bonds of citizenship with you, and to express our hope that you will vote to renew them." It's hardly a blunderbuss broadside of fear-laden No campaign invective. It doesn't mention the pound. It doesn't threaten a withdrawal of love, affection or investment if the vote in September is a Yes.
It is, by its design, a letter signed by people who cannot vote in the referendum, so pointing out that the signatories don't have a vote or don't live in Scotland, while true, is not really an accusation. They are not being cunningly rumbled on that point. Of course, the wording probably went through several drafts. And I wonder what percentage of people approached by its organisers, television historians Tom Holland and Dan Snow, signed it. The list of names does throw up some incongruous matches - Tracey Emin and Bamber Gascoigne, former SAS man Andy McNab and Cliff Richard. I have seen the signatories accused of being rude, "vacuous", "ill-informed" and so on. But who knows how informed each signatory is? (I mean, what does Ruth Rendall know about Scotland? I've no idea: do you?). There is also the presumption all the signatories are English, but I have counted a dozen Scots. I also counted 31 Lords, Ladies and other ermined souls, another reason why the letter may have annoyed Yes campaigners who see Scottish independence as a chance to be rid of the House of Lords and other archaic UK institutions.
There is also the reaction: "Well this is Scotland's business and none of yours." But again, the letter comes from a viewpoint that considers the end of the UK to be its business, too. If you are annoyed by anyone urging a No vote, you will be even more annoyed. The rage does seem a little out of proportion, though, especially given what little effect it will have compared to, say, the grassroots campaigning of National Collective.
I asked Professor John Curtice, the leading poll expert, what practical effect such letters have. "The main result is journalists asking me whether they have any effects," he said. In the actual vote? "I wouldn't exaggerate it at all." It will be greeted by No-ers and quickly derided by Yes-ers. The final poll? Hardly affected, if at all. So, as with most love letters, whether or not the words are themselves convincing, you probably have, in your heart, already made up your mind.