When in a fictional story there are two characters that have a strong similarities/oppositions it's just natural that a part of fans will start to see them related, under a romantic couple context, a rivalry one or other kinds... If tomorrow DoA should include a supernatural character, be sure that we will see easily fan works (of any kind) associating it with Nyotengu, Rachel or Hayabusa, just because all of them share this trait.
In the peculiar case Marie Rose and Eliot want appear the most younger characters of the roster indipendently from the actual age, and youth is associated with passion/love. The correlation is also helped from them forming an actual Tag Team couple even in the game (so having already a real bond), even if I believe that this decision from TN wasn't a cause but rather an outcome of what I said before (originated from such reactions of the fanbase or maybe already in the mind of TN since the beginning).
And finally... everyone is free to think of the characters relationships as prefer xD The fanarts/fanfictions exist even as a way for giving to any fans a concrete way to shape their ideas indipendently from the canon =)