Health maintenance was one; somebody else in this thread has already commented on married men living longer. It works both ways, too. Each partner keeps tabs on the other, notices problems, encourages them to stick to health resolutions, etc. Plus you both are much less likely to forget things or miss appointments, because the other one is there to say, "Didn't you say..." or "Weren't you going to...?" and so on.
There were other, more subtle but still powerful effects. In general, I and my wife, as a couple, were a bolder, smarter, more adventurous "person" than either of us was alone.
So my advice to Rosenstock would be to seek another dedicated introvert. Of course it's hard to get their attention...
[1] https://en.wikipedia.org/wiki/Extraversion_and_introversion
"Introversion is the state of primarily obtaining gratification from one's own mental life". There is a considerable popular literature on the benefits and drawbacks of being introverted.