מִֽכׇּל־מִ֭שְׁמָר נְצֹ֣ר לִבֶּ֑ךָ כִּֽי־מִ֝מֶּ֗נּוּ תּוֹצְא֥וֹת חַיִּֽים׃
More than all that you guard, guard your heart*, For it is the source of life.
The basic understanding of this is one should protect one's heart. Physically, look after it because a healthy heart is vital for a long life. Spiritually, one's heart is the seat of one's emotions and thoughts, which is the centre of a person, and therefore in this sense too, the heart is one's life.
On the latter, I wondered if it could be interpreted two opposite ways. Either it means "protect your heart" i.e. don't open your heart to people unless you really trust them (which is something I believe personally to be a very bedi'eved (non-ideal) solution to preventing damage to one's emotional core, with a lot of negative consequences outside the scope of this question), or it could mean the opposite: don't develop trust issues, as these create barriers around the heart and thus a person will lose their connection to their heart, and therefore they haven't protected it at all. This latter point is my general understanding of how we should treat our heart from my learning.
Either way, best to turn to the commentaries and not only did they go in a different direction, but they also confused me.
The commentaries effectively equate the heart to one's Mitzvot. The message is, treat the easy Mitzvot the same as you would the hard ones, because we see that both the easiest Mitzva (sending away the mother bird), and the hardest Mitzva (honouring one's parents) are both rewarded with "life", which implies every Mitzva is life (Midrash Tanchuma Ki Teitze).
The gemara Yerushalmi states:
רִבִּי אָחָא בְשֵׁם רִבִּי יִצְחָק. כְּתִיב מִכָּל־מִשְׁמָר נְצוֹר לִבֶּךָ כִּי מִמֶּנּוּ תוֹצְאוֹת חַיִּים. מִכָּל־מַה שֶׁאָמַרְתִּי לָךְ בַּתּוֹרָה תִּשְׁמוֹר. שֶׁאֵין אַתְּ יוֹדֵעַ מֵאֵי זֶה מֵהֶן יוֹצֵא לְךָ חַיִּים
Rebbi Acha in the name of Rebbi Isaac: It is written: "More than all that you guard, guard your heart, For it is the source of life." observe carefully all the things you were told in the Torah, for you do not know from which of them life will come to you.
Rashi on the pasuk:
מכל מה שאמרה תורה השמר נצור לבך (מעבור עליו) בין עבירה קלה בין עבירה חמורה:
From whatever the Torah commanded to beware of, guard your heart (from transgressing), regardless whether it is a minor sin or a grave sin.
Guard your heart from transgressing.
See also this fascinating Midrash Tanchuma.
Can someone help me sort out all of the above? What is the connection between guarding one's heart and transgressing, and/or doing Mitzvot**? Why do we say that "we don't know which Mitzva one's life will come from"? Why did King Shlomo use this lashon to teach these messages? Are there any connections between any of this (or any other commentaries) with my initial thoughts in the first and second paragraphs?
* translated sometimes as "thoughts" as according to Kabbalah/Chassidut, thoughts take place in the heart. However, the heart is also a reference to the emotional centre of a person, so both interpretations are correct (and connected - outside the scope of this question)
** I don't generally have any issue with this connection itself, as it goes with my learning that sin causes us emotional damage (and converse), but this connection doesn't seem to be explained here, just assumed. I would like it to be spelled out to me.
