I think that you have to distinguish between wars in which the outcome is uncertain—that is, wars in which real risk to both sides is present—and wars that are essentially a big guy beating up a little guy.
I agree that the former kind of war would happen with the same frequency if this pen pal idea were implemented—the motivation for such wars need to be large enough to overcome the personal danger that they produce, and so would easily overshadow semipersonal connections with citizens of the opposite side. I think that the latter would very much be deterred, though, because the motivation for such wars can be miniscule, to the point where even the humanization of the "enemy" could be a significant deterrent.
I must admit finding it hard to make the distinction you suggest.
Would the American Civil War be a big guy beating up on a little guy? After all, the North was ten times as populous, and many, many times as rich. Nobody on either side thought the war would last very long.
How about the Romans and the Germanic tribes? Nobody in their right mind thought those Barbarians could stand up against the full might of the Roman Empire very long.
Wars are fought until one side decides to stop fighting. This means that "big guy/little guy" wars and "outcome is uncertain" wars don't seem to be that different. In fact the determining factor would seem to be how easy it is for one side to quit. But how would you know how much it would take to make the other side stop fighting until it actually happens? From the history I've seen, when both sides are very intimate with each other's language and culture -- that's when some of the deadliest, nastiest conflicts take place.
I agree that the former kind of war would happen with the same frequency if this pen pal idea were implemented—the motivation for such wars need to be large enough to overcome the personal danger that they produce, and so would easily overshadow semipersonal connections with citizens of the opposite side. I think that the latter would very much be deterred, though, because the motivation for such wars can be miniscule, to the point where even the humanization of the "enemy" could be a significant deterrent.