To illustrate, if a project team member is unhappy towards the PM due to some personal grudges and that being so after rounds of clarifications, the PM still has to take the initiative to approach that team member and involve him as part of the team.
That member may behave unprofessionally but the PM can't afford to. PM has to be thick skinned enough to engage that team member as part of the team.
Being thick skinned is a tool that the PM may employ to remain professional and objective especially if the Management doesn't discipline that team member or remove him from the project due to whatever reasons. The PM has to remain cautious too of any possible damages that team member may bring to the project. But hopefully, that team member will ask to be removed from the project after not being able to contain his frustrations while the PM remains cool, professional and thick skinned enough to involve him/her objectively.