We all think that MS is difficult to obtain, that it requires you to kill your best friend. We also think that in order to obtain EMS, you need to take the eyes of your brother who has MS. I say that we are all wrong. Why? because MS is a Sharingan, nothing more, nothing less.
Here's how someone really gets MS... They have to need it. Yup, it's that simple. Sharingan is gotten the same way, through need.
Sharingan is awakened when someone with Uchiha blood, really needs it. You might say, "Where's the proof?" Well the truth is right in front of our eyes. Sasuke awakened Sharingan for the first time when he thought that Itachi might kill him. He awakened the Sharingan the second time when he needed to see what Haku was doing, because of Haku's speed. Sasuke's Sharingan gained its third tomoe when he needed to be able to keep up with Naruto.
Obito awakened his Sharingan when he needed to save Kakashi.
Now onto the MS part. Madara and his brother awakened his MS through competition. Basically, they were competing within the clan, for control of the clan. So, how do you beat another Sharingan? With a better one! So they both obtained MS. Then, Madara relized that he was going blind. He needed anothet set of eyes that wouldn't lose their sight. That's how Madara obtained EMS.
More recently, Itachi awakened his MS through beating Shisui. But He didn't fight Shisui just to gain MS. He fought him because Shisui found out about Itachi's mission to kill the Uchiha clan. So how would Itachi beat Shisui "the body flicker/mirage"? With a better Sharingan!
Even more recently, Kakashi awakened MS. How would he awaken MS? He needed a better Sharingan to keep up with Gai who is really fast after he has opened up a couple of gates.
Sasuke's MS is the only oddball, because he didn't need it immediately, but he did need it to destroy Konoha. He also needed a new power to save Karin.
So overall, we are overthinking this whole MS thing, because the characters in th manga only knew of how they obtained MS. They did not take into consideration how others got theirs. So Itachi told Sasuke to kill his best friend, because that's how he got his MS.