Why is Sam II such a dick?

Sam I and Sam II are respectively the fifth and sixth clone to work on the moonbase, so I prefer to refer to them as Sam-5 and Sam-6.

Sam-5 had spent almost three years working on the moonbase in solitude, and that experience had humbled him. Three years is a lifetime for a clone. One could compare Sam-5 to a weary senior citizen, and Sam-6 to a boisterous confident teenager ready to take on the world.

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Joe Satriani - "Always With Me, Always With You"
http://youtu.be/VI57QHL6ge0

Sam 5 (or I) commented on Sam 6's(or II) temper to Gerty somewhere in the movie, after their fight.
He said something like, he went crazy on him and that he needs to control his temper, but also added that Sam 5 now knows what Tess was talking about.
Tess wanted a break from Sam because Sam apparently wasnt the nicest guy around.
His 3 years in solitude changed him in the good way.

I dont think it was weird, its one of the many things that i thought made Sam's life cycle more interesting too.
And i think too, that Sam 6 was more aware of the situation, because he hadnt spend 3 years on the base, he wasnt as brainwashed yet and didnt really have any reason for introspection.
But Sam 6 made some good progress already, he wasnt such a hothead near the end of the movie, obviously seeing Sam5 degrade like that will make you reflect on your own mortality, since his fate would have been the same.

And of course, it is a movie :)

each Sam was viewing reality from his own reference point. Even identical twins will react differently because they each recorded reality from their own perspective.

He's a trouble maker!

Pretty sure ferry mentions minor genetic defects or something in the cloning causes slight differences.