I was asked a similar question by co-workers a long time ago, although I was already married. I told them that marriage is very challenging and one must look for someone with the most interests in common. Since we JWs are so different from mainstream society in our beliefs and practices, it would make it for a much happier union if we married another JW. However, people that aren't so serious about the religion do marry outsiders, then of course they start to complaint about their partner or situation as it makes for a tense relationship most often than if one married another JW.
You know, that goes for just about any faith a person is serious about; serious Jewish do not marry outside of their faith, serious catholics should also marry other serious catholics. Nothing new really.
As Dr. Laura says, and she's so very right about this, "love is not enough".
DY