To put it bluntly, Aizen only cares about himself, and views everyone else, allies and enemies alike, as pawns to further his own selfish goals, though some of these twisted relationships have a level of uniqueness.
Aizen ruled the Espada both through fear and his own remarkable charisma. However, he viewed them like he viewed everyone else: as pawns. In the end, Aizen's callousness, coupled with Ben Tennyson's charisma, caused a number of the Espada, and even some of the Numeros, to defect.
Tosen was fervently loyal to Aizen and would obey his every command without question. Aizen, in turn, manipulated Tosen's warped views of "justice" to make him a willing pawn in the insidious Shinigami's ambitions, feeling nothing towards him and being fully willing to callously dispose of him once he had outlived his usefulness.
Aizen initially believed that Ben could be a key figure in helping him bring down the Gotei 13, and became more and more impressed as Ben showed off the powers of many of his forms. However, he also knew that Ben could become a serious threat to him if the hero ever learned the truth. This turned out to be prophetic, as Ben ultimately did betray Aizen. When the two fought, while Ben was Ultimate Swampfire, Aizen bound him with Kido and sadistically taunted him by telling him to watch "everything [he] fought for crumble." However, when Ben defeated him at Fake Karakura Town using Alien X and revealed his power of omnipotence, he underwent a complete mental breakdown and he angrily asked why Ben would "waste" his powers on heroics when he could easily use them to do whatever he wanted. After Ben simply removes his powers in an attempt to show him mercy, he angrily asks Ben who he is to change his fate, only to be punched by Ben.
Aizen was intrigued by Ichigo, but would ultimately dismiss him as an overall threat to his plans over Ben.
Outwardly, Gin seemed to be a loyal subordinate to Aizen, but in reality, Gin despised him for nearly killing his friend, Rangiku Matsumoto, and planned to kill him. Aizen apparently did not anticipate this, or did not show any signs of it, and was completely shocked when Gin fatally stabbed him.
Aizen feared Urahara's intellect, which he acknowledged as greater than his own. However, he remained confident that he would outsmart Urahara.
Aizen rules the Espada through fear, and cares nothing for them.