Heather Wax summarizes two studies that look at LSM or "language style matching" in happy couples. The studies explore when "the way we talkthe grammatical structure of our sentencesnaturally starts to mirror how the other person speaks":
[T]hey looked at transcripts of speed-dates and found that 33.3 percent of pairs who had an LSM above the median wanted to see each other again, compared with only 9.1 percent of pairs with an LSM at or below the median. Then they looked at instant messages that couples sent and found that 76.7 percent of couples with an LSM higher than the median were still dating three months later, compared with 53.5 percent of those who had an LSM at or below the median. Sure enough, they say, "an unobtrusive measure of nonconscious verbal matching uniquely predicted mutual romantic interest and relationship stability."