What is the source and reason for a couple not to see one another before the wedding?
Jewish Action, Summer 2005 edition, has a "What's the truth about..." column by Rabbi Dr. Ari Z. Zivotofsky on not meeting for the week preceding the wedding. His main point is the lack of old sources for this custom, but he does cite several newer sources and the reasons they give. See there for the details, but the reasons and post-facto rationales offered are:
- to avoid dam chimud (which he cites strong arguments against as a reason for the custom, so I'll ignore it);
- to prevent discussions that will lead to strife (Rabbi E.M.M. Shach; Rabbi Y.S. Elyashiv, who therefore bans communication by telephone also);
- to allow the couple time apart for introspection (Rabbi Binyamin Forst);
- to heighten excitement (unsourced); and
- to prevent premarital relations (unsourced).
And as Rabbi Zivotofsky notes, the last reason should apply only to being alone together, not to simply meeting. Also, see the article for reasons the custom does not exist at all and for rabbis quoted as saying it does not (but also some who defended it).
As always, consult your rabbi for a practical ruling.