根本原因可能是乌里韦觉得桑托斯当了总统应该对他投桃报李，而桑托斯觉得他的支持率（很高，但在下跌）是靠自己赢得的，不是乌里韦给的。
The bottom line is that Uribe thought Santos would do his bidding as president, and Santos is convinced that his (high but declining) popularity is of his own making, not borrowed from Uribe.