Or to put it another way, one person probably won't
be able to satisfy all of your desire for interpersonal contact
Western thought about romantic relationships suggests that this is not the case. It suggests that there exists one truly perfect person that you can achieve flawless communication with, both of you always knowing the right things to say at the right time. This is the imperative behind "happily ever after," driving people to search for this single wonderful true love that two people fall into (in books and movies) all the time. People everywhere want this idea to be the truth because it means that some day, under some circumstance, they'll transform into someone totally happy and never have to fall back to earth.
Just like horniness and daydreaming, it's another manifestation of hope.
I used to hope the same thing, too. I had it all planned out: How we would be able to have a conversation just by looking in each others' eyes; how her voice would always be able to lift me out of blackness; how she would look inside me and tell me things I never knew (but always wanted to) about myself. It would be perfect, I would finally be a real, complete person. We would un-break each other. Nice dream.
Reality is never that elegant though, and its problems are rarely that easily solved. Cliches will tell you (and cliches are always right, you know) that real life is nothing like the movies. I'm with somebody wonderful right now, somebody that means a whole fucking lot to me, somebody I love. And, to be sure, I am much much happier than I was before. But it's not the bliss that all of your influences -- Gone with the Wind, William Shakespeare, the Song of Solomon, etc. -- tell you it will be. Happiness, but not perfection.
The point is that those of us in relationships still need people to talk to and communicate and be open with other than our SO. This is by no means a difficult concept, but society seems to think that once you're involved, your need for friends is diminished. It's not true, though, we need interesting people as much as everybody else, it's just much harder for us to connect with them -- we have less time, we're out alone less often, we can't go on adventures as much, etc. None of that means we don't want to, though, and none of it means we shouldn't want to, either.