How can user get PS

[K1domaru]

This is my theory about Susanoo's development.
First of all i think they main key is hatred.
Here we see Tobirama Senju claiming that:

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Here he explains about Uchiha clan appreciating love like no other clan,but that changes when that person looses someone close to him:

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Remember this 3 pages.Let's continue.

We know that Susanoo is the third ability granted by the Mangekyo Sharingan to those who awaken the abilities Tsukuyomi.

But Susanoo consumes persons life.
To me Susanoo consumes person's mental strength and corrupts his evil brain making the person evil.

Now Sasuke fought with Danzo and he awakened Susanoo with emotions about his older brother Itachi.

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[Fecal Matter]

Itachi isn't corrupted though...

 

[Aya San]

Incoming comments about how you don't need Tsukuyomi and Amaterasu to get Susanoo

 

[Champ]

Awaken the Mangekyo, then if you're lucky you'll unlock Susano'o since Obito said that ability is rare. Then obtain the Eternal, become proficient with using it then achieving Perfect Susano'o won't be a hard step to climb

 

[VongolaX]

Itachi had none of the above yet his susanoo reached the final stage of MS Susanoo.

Once you transplant to EMS, it's an automatic PS