I think that motivations could be of a variety of natures and if they are not aligned tensions will surely arise.
The typical example is when researchers “use” PPI as a way to get their research funded, driven by the need to impulse their careers, giving false hopes to patients they are involving. On the other hand, I think that a very “passionate” patient, who is either too focused on his/her own life experience and can’t extrapolate it to a wider view of the condition, or who wants to impose his/her points of views to the research team won’t be very helpful.
About people with long-term conditions or people in need of an immediate answer, I think they may become frustrated when things don’t go as expected or promised, when a project they have been working on does not get funded, when study results are not as expected, etc.
In my opinion, when everyone is on the same page and each of the members of the team know their roles and responsibilities, when things are planned and expectations are set from the beginning, it is then when research benefits the most. I believe that educating the research team and P/P on many aspects of this interaction could avoid lots of false expectations and bad experiences.