I'm not a Minmatar but I will tell you why.

Originally after the EVE Gates collapse, a massive wormhole in space, so I believe it to be, all of the scattered settling races of New Eden were thrown into their own Dark Ages. The Amarr were religious missionaries of sorts, the MInmatar were more fractious.

The Amarr home world of Athra was a desert planet and a tough one, the Matari planet was a natural paradise.

Because of this the religious Amarr considered their Dark Ages a test from God, which they passed, formed their own religious. Additionally the Amarr were the first peoples in New Eden besides the Jove to take to space, and with their religion they formed one core tenet that was their purpose in the universe.

To Reclaim

"I give to you the destiny of Faith,
And you will bring its message to every planet of every star in the heavens:
Go forth, conquer in my Name, and reclaim that which I have given."I
- The Scriptures, Book of Reclaiming 22:13

In doing this the Amarr set out from Athra to Reclaim the stars in the name of God, the Holy Empire, and Mankind.

They began to conquer and enslave all of the races they encountered, subjugating them and testing them, and finally allowing them to enter the main body of the empire once that race had proved themselves worthy. This happened to the Ni-Kunni, Khanid, and several other subservient races.

Then the Amarr met the Minmatar.

Depending on who you ask, the Minmatar were either without a means of space travel, or were already colonising other planet, I believe they were using a very crude form of space travel, however the Amarr set upon them and conquered them easily with vastly superior technology.

The Minmatar were now a slave race.

What you have to understand about the Amarr is that they don't take slaves for personal gain, to get free labour while they relaxi and the slaves die, nor do they do it for any other reason than to test the slave races to see if they are ready to join the empire.

In anycase for several hundred years the Amarr made the Minmatar toil away, they gave them educations, technology, taught them, schooled them, loved them as a parent loved a child.

However Amarrian Imperial dreams were growing too greatly, the the Federation fearing a war on a second front (they weer fighting the Caldari) would intervene.

The Gallente needing something to stop the Empire from attacking them devised a plan to use the Minmatar as a shield against the Amarr. They supplied the Matari with weapons and a means to fight back. Meanwhile the Amarr arrogantly challenged the Jovians in order to enslave them too, broadcasting their intent weeks in advance.

The Jove crushed the Amarr fleet, the MInmatar rebelled. And millions died in the Great Rebellion. The Minmatar were split in two, the loyalists took the name Ammatar (Amarr and Matari combined) while the renegades took the old Minmatar name. However they could not liberate all of the slaves.

Traditionally a Matari warrior would say he is fighting for Freedom for his people (an Ideal inspired by the Gallente), Revenge against the Amarr for the enslavement of his people, because they have something to prove. Under the surface their is a lot of bitterness against the Amarr in some cases unwarranted ( the chronicle where a bitter Matari man cannot understand why the Ammatar love being a part of the Empire, in the end he kills his own people because he is bitter).

Now for my propaganda.

While the newly freed Minmatar enjoyed social freedom, they became a poor empire, somewhat subservient to the Gallente, many Matari leave as refugees to the Gallente to face tough social barriers, in some cases discrimination, in others they plunge to the bottom of the Gallente social structure to live in squalor and hardship.

While the Ammatar who remained loyal, now have schools and hospitals, are cared for by the Amarr, are not mistreated or abused, who choose to believe deeply in the Faith. These Ammatar are now a part of the Empie, somewhat and live in relative peace.