In an NFL Network discussion last night, they were spitballing the relative merits of staying in DAL or going to PHI.
One school of thought is staying in DAL is best for his "brand" and recognition, they are one of the highest profile teams, he could lead the league in rushing again, etc.
Another was that, if he can make about the same or even a little more, sharing the load with Mathews, it could save wear and tear on his body and extend his career. I can see that rationale. We don't know his state of mind, but he may be disappointed DAL didn't step up to the plate more, when he was the engine that drove the offense last year. Great OL, no doubt, but not sure they can just slap another RB in their and expect the same results.
* That answers why Murray might want to go to a team with Mathews, not necessarily the other way around, but the same rationale could apply if he gets paid the same?