First, let's step back from the speaking of God's Name to the writing of it. Why is it many observant Jews even refrain from writing "Lord" and "God," but render them "L-rd" and "G-d" instead? (I've even seen a few Messianics take this to an extreme, writing "M-ss--h" instead of Messiah.)
The answer is found in Deu. 12:2-4:
Ye shall utterly destroy all the places, wherein the nations which ye shall possess served their gods, upon the high mountains, and upon the hills, and under every green tree: And ye shall overthrow their altars, and break their pillars, and burn their groves with fire; and ye shall hew down the graven images of their gods, and destroy the names of them out of that place. Ye shall not do so unto YHVH your God.To understand why a Jew will not write the Name, or even a title, of God, you have to look at this passage like a rabbi. Remember that the rabbis both seek to keep the most literal interpretation of a command possible as well as observe its drash. For example, when an Orthodox Jew wears teffilin (phylacteries) in prayer, it's to keep the command to wear God's Word on his hand and between his eyes literally (Deu. 6:8). Therefore, when they see a command to destroy the names of the pagan gods, but not to do the same to the Name of YHVH, the observant Jew likewise takes that command very literally. If you write YHVH--or indeed, any Name or title belonging to the God of Abraham, Isaac, and Jacob--on a piece of paper, and then either erase the Name or allow the paper to be destroyed, to the Jew you are destroying the Name of God.
What then of speaking the Name of God? Forbidding this practice came out of two separate issues. The first is the command that he who blasphemes (slanders) the Name of YHVH must be put to death (Lev. 24:16). Again, think like a rabbi: The noblest pursuit in their minds is to put a fence around the Torah--that is, to erect commands beyond what the Torah commands so that one will not accidentally sin. For example, to create a specified "Sabbath's day journey" that one isn't supposed to walk beyond (about half a mile). If one were forced to walk just a little more than the prescribed journey, one wouldn't have sinned against the Sabbath by working. The same principle applies here: The simplest way to avoid accidentally blaspheming the Name is to avoid using it altogether.
The second issue is that the pagans in the first century (and the neo-pagans of today) used the names of their gods in magical rites, and the Jews didn't want them to use YHVH's Name the same way. This is why, for example, the book of Esther only contains the Name in four hidden acrostics, and then only in the original Hebrew: It was a safeguard against the Persians, among whom the book was published (likely in their own language) learning and misusing God's Name.
This resulted in an increasing sacredness in the use of the Name of YHVH. First, it was restricted from common use, with one substituting Adonai (Lord), Avinu (Our Father) and other circumlocutions instead. By about two centuries before Yeshua's birth, this practice had been enshrined in what some call the ineffable (Unspeakable) Name doctrine. The use of the Name became restricted to the priests, and then to the Cohen HaGadol (the High Priest), and then only on Yom Kippur. Edersheim notes that where once the practice was to say the Name aloud on Yom Kippur, when it became known that the Name was being used for magic, the Cohen HaGadol began muttering it under his breath, until the very pronunciation was lost from the common mind.
In my previous entry, I showed that the Bible does actually command us to make YHVH's Name known. This of course means more than just the syllables--it means His reputation, His honor, who He is--but it includes the syllables. But now I'm going to issue a caution: Yeshua and His Apostles were very careful about using YHVH's Name. Yeshua most commonly referred to Him as "My Father," and the NT uses Kurios (Lord) and Theos (God) rather than transliterating YHVH into Greek. Therefore, we too should exercise the greatest of caution in actually speaking God's Name, doing so only in worship, prayer, instruction, or another reverent context.
We want God's Name to be known, not for it to become common.
On to part 3.