According to the WIkia page:
Theories of creation of doppelgängers
There are two leading theories for the existence of the known doppelgänger line, both polar opposites, that might explain it. There hasn't been much conclusive proof regarding either of them, though:
- Hybrid curse caused the line:
- According to this theory, because Klaus was cursed with the hybrid curse, nature devised a way for the curse to be broken by letting a doppelgänger be born every 500 years.
- By this theory, Tatia was a random ingredient and was not a doppelgänger herself.
- It also means that Klaus' existence is the reason for the existence of Katherine and Elena.
- The reasoning behind this theory is based on Elijah's story of the original Petrova that involves Estherusing Tatia to end the feud betweenKlaus and Elijah.
- This theory doesn't give an explanation as to why Klaus believed he could make hybrids every 500 years after the curse was already broken.
- It also doesn't explain how Tatia's bloodline could have contained doppelgängers, as her child was already born before she was used in any of the spells.
- Tatia was a doppelganger:
- By this theory, doppelgängers are not a consequence of the hybrid curse but they have existed in the world long before Klaus' curse.
- The process of creation of doppelgängers is unknown; yet it postulates that they don't simply appear for breaking curses, because the intervals between their appearance is beyond the lifespan of all species except immortals.
- According to this theory, Tatia is a doppelänger herself, probably a descendant of a long line of doppelgängers before her.
- Esther's use of Tatia, in turning her family to vampires and cursing Klaus, is not a coincidence as doppelgängers are a rare occurence and reversing spells involving doppelängers is quite a difficult task as they appear in large intervals. By extension, Tatia was not used to end the feud between Klaus and Elijah, as the latter believes.
- If Tatia wasn't special to begin with, there was no reason for Esther to use Elena for rituals - linking her children, and remaking of Alaric.
- Esther mentions that as the doppelgänger, Elena's blood is a potent binding agent, which incidentally is the same role herbs play in spells. This means doppelgänger blood are not any special except for the fact that they're rare.
- This theory allows doppelgängers to be born despite breaking the curse as the existence of the Petrova doppelgänger had nothing to do with the curse.
- It partially allows the idea that doppelgängers from other lines could be used to transition Klaus' hybrids.
- It can explain the passage of doppelgänger aspect in Tatia's child and Klaus' ability to transition his hybrids if Elena's line had continued.
Ok, so, as stated in Graduation, Silas was truly immortal, so to maintain balance, there needed ot be a version of him which exists as a mortal; in other words, there needed to be a version of him which could die. That is why Stefan exists as a doppelgänger.
This could similarly be applied to the Petrova Doppelgängers, as currently we have no clue of how they came into existence. It could be that the progenitor of the line became immortal and that is why Tatia, Katherine, and Elena exist. So, possibly Silas' Lover was the originator of the Petrova bloodline. This might also explain why Katerina and Elena had previously chosen Stefan over Damon (but that's possibly a fluke, considering Elena is currently with Damon).
Any other ideas?