Hi, great posts here.
I believe that we use the word passions somewhat variously. On the one hand, we use the word "passions" to convey topics, objets d'art or mental pursuits in which we invest emotional and intellectual currency. On the other hand, we might use it to suggest deep and perhaps uncontrolled emotions.
If we mean it in the first way, and I hope we do, then I think it simply means the growth, serenity and perspective that we can acquire from becoming deeply involved in something. Whether that's macrame or Michelangelo, it allows us to begin to see patterns both in the object of our attentional affection and, by extension, in other, related things. It is often through the intense focus on one thing that we can begin to see that thing's interrelatedness with other things. Robert Persig's famous book, Zen and the Art of Motorcycle Maintenance is about gaining trust and closeness with someone, in this case the author's son, through their sharing of the work of maintaining a motor vehicle. Detail, balance, observation, the fitness of parts for their purpose, the importance of a disciplined outlook, the beauty of successful design - these are just a few things that really getting passionate about something can tell us about an object, and about many other things.
For me, recording and mixing music in my home studio is a way I really get focus and a sense of accomplishment. Just as a painter usually does not live in a cut and dried, black and white world, so it is with music; instruments and voices must be balanced just so - there are no hard and fast rules. It's different every time, yet requires the experience that doing it a lot gives one.
Those of us who follow our passions have come so far for beauty's sake that we dare not backtrack. We are transfigured, and there is no relearning what we have abandoned along the way.