There's many reasons:1. eye contact2. when you walk around he may brush your shoulder with his.3. he may ask you question alone or ask when you were born or when your birthday is.4. he might ask...
Yes, if you are a girl marrying an 18 or older (I don't know about the other way around), but be very careful that you do not marry for that reason only. This may sound old-fashioned and fairy-tale...