The physicality on a mentally weak player would take its toll.
I love how everyone keeps calling LeBron mentally weak. Mentally weak players don't go to 7 straight NBA Finals, propping at least 3, possibly 4 (at least 2, possibly 3 Cavs teams and the last Heat team), of those teams squarely on his shoulders and taking them to the Finals kicking and screaming.
But hey, when you run out of legitimate reasons to hate him, I guess just call him mentally weak. Whether it's accurate or completely not.