Exactly What It Says on the Tin
. A couple, married, dating, or "just friends," happen to have names that start with the same letter. Let's face it, it's cute...or it's supposed to be (it may or may not be by the writer's design). This trope also tends to occur with non-romantic best friends as well.
Please note, however, that this trope only counts if the couple in question consists of major characters, and the relationship is ongoing. Boyfriends/girlfriends/former classmates/etc. seen in one episode and never heard from again do not count.