It's been fairly well documented among doctors that depression, anxiety, and the like suppress the immune system thus opening up the body for all kinds of things. I'm not just making this up. With my own health issues, medical information I received said this very thing. Of course, I suppose someone could "run" with this and say all one had to do was "stay happy" and good health would be guaranteed. That's not true either, but it is a factor.
My in-laws have been married for 65 years! He's 93 and she's 86. Every night before going to bed they sing love songs to each other! I live with them and I know.