Are personality clashes with other faculty a valid reason to deny tenure
In practice, "collegiality" is often a consideration in tenure decisions, whether or not it is a factor identified in the university policy on tenure and promotion.  

The American Association of University Professors (AAUP) has come out with a statement against the use of collegiality as a an explicit factor in tenure decisions:

http://www.aaup.org/report/collegiality-criterion-faculty-evaluation/ 

However, this report also makes it clear that a faculty member who is hostile to the other faculty in his/her department would typically not be doing an adequate job in service and teaching, because working with colleagues in these areas is a necessary part of the job.  

Thus "We're not granting tenure to John because no one likes him" wouldn't be a good reason, while "We're not granting tenure to John because he doesn't work productively in departmental committees" would be a reason to deny tenure.  

As a practical matter, if the tenured faculty in the department don't like you and don't want you to get tenure, they'll probably find an excuse to deny you tenure.  Any tenure track faculty member would be well advised to get along with the other faculty in the department.