At various times in my academic career, I have proposed a research project to a colleague or a graduate student, but the project has either not captured their interest, or the project has been deemed impossible to do or not worth doing. That is fine. I am perfectly willing to face the fact that not all my ideas are interesting, feasible, or sane (or that I am easy to work with).
The main topic of this post involves situations in which I went ahead and did a project anyway -- with different colleagues or students -- and then the original, dismissive colleagues/students get upset. Perhaps I could have worked harder to 'sell' them on the project, but if someone says to me "That project can't be done" and is not impressed with my arguments for why/how it can and should be done, I am going to work with someone else. It can be good to work with someone who makes you reexamine your ideas and forces you to make a compelling case for the research, but the collaboration will only work if the other person does something other than seeing the negative side of everything and creating unnecessary obstacles.
In one case involving a student at my previous university, I spent considerable time getting the student started on a project so that he could get far enough into it to see that the research could be very interesting and significant. After a year, he decided it was "too difficult", involved too much tedious work, and was not worth his time. He moved on to another project and advisor, and I did the original project with an undergraduate who completed a senior thesis, published a paper, and went on to success in graduate school and beyond. A couple of years later, the first student told me that he was upset that I had not invited him to return to the project once it was further advanced and was clearly going to lead to something. He said "So you wanted to keep the glory for yourself?". It was clear from discussing the situation with him that he remained unconvinced that it wasn't somehow my fault that he ended up with a dead-end project with another advisor.
I know that some students might not have the necessary perspective to make a decision about which dissertation project will have the highest impact, but at the same time, I am not going to force a student to do a project that they say they hate.
Similarly, a colleague I approached about an interesting project thought the project not worth doing. It was not just a casual dismissal of the project -- we had a number of conversations about it, and the colleague did a lot of background reading and thinking before deciding the work couldn't be done. Other colleagues and I did the research anyway, and it turned out to be extremely interesting (even more interesting than I first imagined). The colleague who dismissed the project is now upset that I did the work with others, although we remain on friendly terms.
How hard should I have worked to involve this person before deciding to work with others? How hard should I work to convince a student to work on a project that they are inclined to reject?
Sometimes I get asked by others to work on projects with them and I say no, most typically because I don't have time. I really hope that if I say no because of erroneous pessimism about a project, that I would know that I had let slip a good opportunity and would feel wistful, but not bitter, about it.