>>42675427 (OP)
The episode heavily implies that she was treated this way by someone else when she first went to Manehattan and adapted by continuing the cycle. Rarity not letting Suri's actions let her fall into the cycle of bitter isolation is the message of the episode, that just because someone takes advantage of you doesn't mean that it's justified to inflict that treatment onto others.
>>42675427 (OP)
She was a naive small town girl who was broken by the big city. Tale as old as time. She was probably really nice back when she lived in Ponyville.