How does Caesar feel about Cassius and why does he feel this way?

Caesar feels suspicious of Cassius and somewhat apprehensive, although Caesar twice denies that he is afraid of him. Caesar explains to Antony in detail what he thinks of Cassius and how he feels about him. Caesar shows himself to be a shrewd judge of character, because his appraisal of Cassius is close to what we have seen to be the truth. Cassius is already plotting to organize a group of similar-minded men to assassinate Caesar.

Would he were fatter! But I fear him not,
Yet if my name were liable to fear,
I do not know the man I should avoid
So soon as that spare Cassius. He reads much,
He is a great observer, and he looks
Quite through the deeds of men. He loves no plays,
As thou dost, Antony; he hears no music;
Seldom he smiles, and smiles in such a sort
As if he mock'd himself, and scorn'd his spirit
That could be moved to smile at any thing.
Such men as he be never at heart's ease
Whiles they behold a greater than themselves,
And therefore are they very dangerous.
I rather tell thee what is to be fear'd
Than what I fear, for always I am Caesar.

Caesar is certainly right in thinking that men like Cassius are very dangerous. He feels this way especially about men who read much, are great observers, and who look quite through the deeds of men. Caesar is especially sensitive about having a man like Cassius look through his own deeds because he has been directing all his efforts towards the goal of becoming the sole ruler of the Roman Empire. He is intensely ambitious, although he tries to hide his ambition from others and is successful in doing so with most people. However, he knows that Cassius can see right through him. Caesar can't fool everybody. Cassius understands him through and through. Cassius is "very dangerous," therefore, in being able to inform others about Caesar's intentions and probable actions. And, of course, Cassius is dangerous in being capable to plotting to have Caesar killed--which he does. Cassius seems to want Brutus to become a member of the conspirators partly to lend them a facade of honor and patriotism, but also to allay Caesar's suspicions and his caution. Brutus is Caesar's best friend. Caesar would naturally think that anyone who was a good friend of Brutus could not be a secret enemy of himself. Cassius is not only disliked by Caesar, but he is not well liked by anyone. He knows he needs someone like Brutus to dignify and justify the assassination he has in mind.

There are many people like Cassius, and we can meet them today. One striking characteristic which Caesar notes about such men is:

Seldom he smiles, and smiles in such a sort
As if he mock'd himself, and scorn'd his spirit
That could be moved to smile at any thing.