This proverb puzzled me when I read it. It sounded to very Buddhist; like passion is inherently bad. A problematic position when it was written of Jesus that zeal (passion) for God’s house consumed Him. God says in multiple places that He is jealous (same Hebrew word in both those verses and this one … I checked). So, there must be a right way and a wrong way to be zealous/jealous/passionate. There is a right way to be angry — Be angry and yet do not sin (Ephesians 4:26) — and a wrong way to be angry — letting the sun go down on your anger, same verse — so it is reasonable enough to conclude that there is a right and wrong way to be passionate/zealous. For many things that we drop solidly into the good or bad camp, God tells us that there is a right way to do it and a wrong way to do it. Passion is, I think, one of those things.
What is more, the context of the words that precede it give some insight into how it ought to be understood. The word used for tranquil could also (possibly better) be translated as sound, like the whole sound mind and body thing we hear every time someone makes a will in TV or movies. To be sound is to be intact; whole; not lacking anything necessary; healthy. Juxtapose that with the possibility that passion could also be rendered as jealousy or envy and you have a pretty strong case that the verse could probably better be rendered as:
Anyone who has suffered from jealousy can bear out the truth of this proverb. Jealousy eats away at a person, undermining physical and mental health as well as friendships. By contrast, the person who is sound in the essence of who they are (heart), has none of the psychological or physiological problems that the envious person has. It is not that the person of sound heart is healthy as a horse at all times, but that the maladies precipitated by envy are not present. It is a warning with a promise; a warning that my heart impacts all of me and that jealousy is a cancer that will destroy me.
This verse is challenging. Marketers would like me to be discontent; not sound. Jealousy is one weapon in a large arsenal that they bring to bear in their attempts to get me to buy more stuff — whether I need it or not. On the other hand, being a sound person in the inner man means I can shake my head at those attempts and know that God provides absolutely everything I need. The world’s systems — of which marketing is only the most obvious example — push me to be unsound so that they can tell me what I need. God challenges me to be sound; complete; lacking nothing. And His challenge is valid, because He gives the challenge with a promise that He will supply all my need according to His riches in glory in Christ Jesus (Philippians 4:19). Will I be sound or envious? Will I worship the true and living God or the old Invidia? The choice is mine.