There are some cases where being brutally honest has no long-term benefit so you might as well white lie, but it takes too much mental overhead to figure that out at every such stage in every conversation. I hate it when people default to always white lying.
I default to being honest, to the extent that I respond truthfully to the dreaded "does this make me look fat" question. I hope we can still be friends.
This is the heart of the matter, I think. A large part of the brain's power seems to be tuned for sensing status and other social friend-or-foe questions. It's a genuinely complex problem, and it's hard to do while you're also thinking hard about something else.
(The flip side of this is that sometimes it's useful to anthropomorphize the elements of the problem, and that explaining a problem to someone else often brings out aspects you've overlooked.)
And maybe people like us are broken, in a sense. It feels like I'm missing something everyone else around me seems to have: an ability to immediately understand the reason why people phrase things the way they do.
The person you're referring to probably knows he has the problem and has forgotten about it. If you want to point it out to him, pretend like you're pointing out an embarrasing bad habit, like picking his nose. So you probably want to tell him in private, etc. Also he'll probably deny having the problem. Let him deny it. Just tell him specific examples of what you've noticed. The more specific, the better. A video would be ideal. (Not that I think you'll get one; I'm just trying to convey that telling him specific examples is very important, because otherwise he won't be able to make the mental leap "I have a problem" => "Here's how I fix it".)
Buddy: I did bungee jumping last summer, did you know?
Me: "umm... all right?"
...
uncomfortable silence
Faking enthusiasm is something I find very difficult to do, but I guess I should at least start trying.
Also, how to stop assuming that other people are not sincere when they show interest to me?
I have struggled with this in the past. The key takeaway I've learned is that the person is just trying to relate a feeling to you - about what they are talking about - e.g. not to convince you. Stop trying to be a know-it-all or Mr. Logic.
Example: Woman friend enters wearing thrifty-looking but interesting pea coat.
Friend: "Hey, check out my coat. Guess how much I paid?"
Me: "Hmm. $25?"
Friend: "No. $30"
Both: Silence
What I should have said: "$100?!"
Or even better?: "Let's go case out that store. I want one in my size." (joking)
It's not about getting the correct answer - this is not a game show "The Price is Right" - being friendly is more about letting someone express their emotion behind their words and maybe even brag a little