Just because you won't, doesn't mean they won't.
There are a few main categories of costs associated with everyday decision-making:
Everyone has their own limits for each that must be calculated for every situation that pulls from that category's coffers. (I view this is different from an opportunity cost.) Only if it is below the threshold for each can the action be done. For example, I may be willing to put up enough money and time for the glory of summitting K2, but my death risk tolerance is much lower than its ghastly 13-23% fatality rate, so I say no.
This is straightforward and easy to do for a single individual, but becomes much more opaque when more people are involved, especially when only one person has all the information.
John goes on an African safari with Clarence on a whim, doing little research and spending $10,000 and 10 days of his precious PTO. He trusts it will be awesome based on Clarence's previous trip reports. He is excited for constant action with animals and the local culture. He quickly realizes that seeing the "cool" animals—lions, leopards, and such—is not guaranteed and that most days are coupled together doing the same thing in case something isn't seen the first time (e.g., days 2 and 3 are the same, 4 and 5 the same, etc). He would have spent at most $5,000 and 5 days of PTO, not $10,000 and 10 days.
John underestimated how much Clarence values seeing these animals, even if only for a short amount of time. In turn, he underestimated the cost Clarence was willing to incur to make this happen.
Brett happily agrees to go out for a "nice dinner" with his friend Samuel despite not being very into food or drink. Samuel picks Brett up and they carpool to a "pretty good spot" where a valet whisks the car away while the two men head in. Brett is astounded at the prices, estimating he'll have to spend around $150 to not look stingy in front of his new friend—a nice dinner to him was at most $50! Meanwhile, Samuel is salivating over the menu and ripping off questions to the waiter about umami this and confit that.
Brett underestimated how much Samuel likes to fine dine and had to pay for it.
Amy accepts an invitation from her friend Sonia to help her prepare her front yard for a garden, something she only cares to do because she is a good friend. They get started early on a Saturday and Amy soon sees just how laborious the day will be: Sonia wants it to be perfect. Not just weeds pulled and plants planted, but trellises installed, rocks removed, trees pruned, mulch replaced, and decorative stepping stones arranged. The time quickly ticks while actual progress feel slow and sweat-soaked shirts weigh heavy. They finish late with blisters, sunburns, and plenty of aches all over.
Amy underestimated how uncomfortable Sonia was willing to be in order to see the garden of her dreams come to fruition.
Ketanji and Stephen are starting theid dream company despite knowing it's going to be a long, arduous process with a low likelihood of success. The first months are filled with regular 100-hour weeks and constant pressing from investors who want a return on their seed money. The stress starts to pile up. Both founders' health begins to deteriorate quickly from lack of sleep, stress, and other unhealthy habits. Ketanji asks Stephen at what point he's willing to give up to save his mental health; he looks at her puzzled and explains that he was always expecting and willing to endure conditions like this.
Ketanji underestimated how much suffering Stephen was willing to endure in order to see his dream company come to fruition.
Elena has signed up for a Titanic exploration expedition. She trusts it will be awesome based on it being the freaking Titanic (how much cooler does it get than that?) and safe based on the company's previous dives to the sunken ship being successful and the CEO, Neil, choosing to go. Unfortunately, she didn't have time to realize her mistake as the vessel imploded, killing her instantly.
Elena underestimated the Neil's risk, and thus cost, tolerance. She gave a "0.1% risk of dying is acceptable, but nothing more" and assumed Neil had a similar number given he was also diving with them, when in reality it was much higher.
(u/Sol_Hando has an excellent comment here that this is based off of. See also Gwern's reply.)
All of these examples come from the perspective of the underestimator (UE), but can easily be reversed to be seen from the overestimator (OE).
The false consensus effect is real. The cost—time, suffering, money, literal death—one is willing to bear is unlikely to match someone else's whether more or less. The only way to be certain is to quantify and or qualify the cost. The "ugh" party in all of the above examples would have been substantially better off had they simply asked for more details and adjusted according to their preferences:
Additionally, friction and resentment is often created between both people, where the UE is annoyed at having to spend so much X or the OV is annoyed at the UE not spending enough.
The solution to prevent unfortunate occurrences like these is to provide or request as much information in all cost fields as socially and reasonably possible. Rough, non-weird probabilities can be given as extra info in the form of words of estimative probability.
Here is a non-exhaustive list of questions to ask for each cost category when not all information is readily available:
Those who have the information should offer up as much of it as possible, which can include proactively answering the questions above.