I don't think he is a total outsider to Singania and Aluwalia past
we know that something happened in the past with Aluwalia's family
and Daadu doesn't like taking about it
Arohi's dad once kinda brought it up saying that once more cos of his daadu's suty they are risking family happiness...or something along these lines
and Daadu told him to be quiet
we also know that something happened with Singania due to Aluwalias...something he has not be able to forget and he once said that he will tell Arjun when the time comes
I'm getting a feeling that Arjun is also related somehow into this mess
maybe his real parents died because of the feud between these two family
the notion of family being really important to Arjun and Arohi is very significant
and the fact that Daadu is constantly bewildered with Arjun's actions
i think that Arjun's past is also related to Singania and Aluwalia past
what do you think?
what are your predictions for his past?