In Shippuden episode 181, Sasuke complains that his missions aren't tough enough and he wants to be given more challenging missions in order to improve his skills. He comments, "I don't see how these silly missions are supposed to improve our skills."

Kakashi replies, "The purpose of these missions isn't to improve your skills. You're not doing this for training."

With that reply, Sasuke says nothing but is annoyed and looks away, implying he doesn't agree and he still wants tougher missions.

This exchange highlights the difference between Kakashi's and Sasuke's outlooks. Kakashi sees missions as the endgame: ninja accomplish missions because they are paid to do so, and completing missions successfully is all a ninja needs to do. For Kakashi, accomplishing missions is the purpose of a ninja. A ninja need do no more than that, which is why Kakashi is not bothered or frustrated by easy missions.

Sasuke's outlook on being a shinobi is quite different: accomplishing missions successfully isn't the endgame, it's just a stepping stone to a much bigger goal. Missions are merely practice and preparation for a bigger, more important goal, which for Sasuke is killing Itachi. Sasuke is only interested in missions because he's interested in getting stronger, and he's only interested in getting stronger so that he can kill Itachi and thus complete his life goal and purpose.

Sasuke views missions merely as practice for the real thing, not the real thing itself, as Sasuke's goal is revenge. Sasuke later even says that becoming chunin "means less than nothing to me" because Sasuke cares so little about the formal structure of the shinobi forces. Sasuke only cares about gaining the strength and capability to accomplish his goal. Missions don't interest him unless it involves a chance to test and sharpen his skills, so Sasuke is ever eagerly seeking out new challenges to test his skill, and is frustrated and impatient when low-level missions don't challenge him at all. 

In short, Sasuke views missions as a means to an end, and for Kakashi, missions are only the end.