This is on strict rationalism. This is not coming from an emotional place, it's coming from a questioning place. Sorry for the very "Robin Hanson" style posts today.
I've always thought it's strange that people try to make me feel better on a bad day with "Well, if it's any consolation, (insert their bad news here)".
Your misery is uncorrelated with mine, and is strictly negative for me if I like you. How is that supposed to make me feel better? Making me feel like I'm not alone is stupid, because I WISH I were the only one in the whole world having a crappy day. I'm sure Robin Hanson would have something to say about the anthropological background to this, perhaps a reason that may be strong enough to counteract my "strictly rationally, this is stupid" reaction, but I'm guessing that whatever effects he'd bring up would be secondary to the primary logic. The intention of them saying it is clearly good - they want to signal that they understand my pain, and to help me feel not alone - and that intention can make me feel good, but isn't there some better way of signaling that intention without a negative factual?
Another example (which seems to me to be intuitively related, but I'm not quite sure how) is that it's rationally strange that people don't put "single" on facebook when they're single and would prefer a relationship. It should not be embarassing being single or wanting a relationship (these are part of being human in modern society), and reducing search cost should be a good thing for people with whom you may be compatible.
I understand the signaling value of looking like you could be in a relationship, but the person you're eventually getting with is going to find out anyway.
The only explanations I can think of is either a knee jerk preference for privacy (which I doubt, given the amount shared by many people online), or a "commitment and consistency" thing where people have to work to figure out if you're single, which increases your "mystery" and makes people want to act consistent with the initial effort they've put into figuring out if you're single - that you're worth time.
Relationships are hardly rational, so it's not quite so important to adhere to rationalism when seeking for one, but I know my initial reaction to someone who is single but doesn't have "single" up is that they don't really want a relationship and won't really put effort into finding or maintaining one - something strictly negative. My next reaction is "they're too self-conscious and don't have any confidence", and my next one is "well, if neither of those are the case, they think very tactically/politically". None of those three things should be things you want a single person you could be interested in to think. I don't know if this overpowers the commitment/consistency motive. Perhaps it depends on the person you're trying to attract - if you want someone very commitment/consistency oriented, it's good, whereas if you want someone adaptable, it's bad. I've always associated adaptability with intelligence, so if you want someone commitment/consistency oriented, it means you value bullheadedness. That means I still shouldn't really want you.
A final example would be the extreme negative reaction of some people to hearing someone else say "I love you." "I love you" is a factual statement and has two meanings to a listener: 1) the speaker claims to love the listener and 2) there are emotions that person is feeling that has led them to say "I love you".
Both of those things are good things for the listener - being loved is good, and having emotional attachment is often good. Sometimes, perhaps, a negative reaction can come from a feeling of "Oh no, I don't feel the same way to the same extent and I will have to hurt this person" or even "Oh no, I'm going to get attached and then they will hurt me one day", but I think we all know couples whose relationships that were going along wonderfully and had both people excited about the long term until one person said "I love you". This negative reaction by the other person is not a "rational" one.
This isn't to be judgmental - people are welcome to do or think what they like - as long as it doesn't affect anyone else directly, it doesn't matter to me, and people should do what makes them happy or comfortable. There's a difference between judging someone for being a weak person and judging someone for being a person. But these are three behaviors that require "irrationality" to make sense. More constructively, this isn't a statement of "people are irrational and that's stupid", it's a question: "How can I, as an imperfect and irrational person, 'practice' being rational, and when will it improve my behavior?" In these situations, that should be a good thing - for being happy, for finding love, or for nurturing it.