A recurrent theme in Roman political thought inhereted from the Greeks, as almost everything of theirs was, was luxury and the possibility for luxury to corrupt. Specifically to corrupt a person's public spirit or the general aim of perfecting themselves (the Greek notion of arete). So Caesar was cautious of the potential contained in evidently ambitious men who were not letting themselves go in luxury as they were potential rival tyrants, or they could be those who wished to restore a more Athenian mode of governance... It turns out there were plenty of both types around.

A related notion from the ancient world:

http://en.wikipedia.org/wiki/Tall_poppy_syndrome