I think it means that someone might be overly attracted by someone whom he thinks is right for him.
Personally I'm not familiar with the expression, "be off on someone" in that context.
From the rest of his statement, it is clear the author is advising people to be very open about who they connect with, and to introduce yourself even to people you don't immediately think are "right" for you.
This makes me think his intended meaning of "to be off on someone" is "to be preoccupied with s/o", or "to be chasing off after s/o". He cautions that you may be likely to chase off after someone who you think is right for you [... but you may overlook someone who actually is right for you].
Judging from other people's comments here, I wonder if this expression might be rare, regional, or even the author's personal invention.