Don't use the formal 'Lei' with me
my knowledge of Bach is not comparable to that of a pianist; I am a singer and, alas, Bach is still rarely sung in solo teaching, choral settings, and even less so in professional solo and choral performance.
But I want to try an experiment. Instead of trying with Bach arias that I already know, I will download a new one and immediately try to make you understand what I mean.
The first sacred cantata I found in the list of Bach's works on IMSLP: Ach Gott, von Himmel sieh darein http://conquest.imslp.info/files/imglnks/usimg/b/b3/IMSLP24170-PMLP03250-bwv002.pdf
The aria is on page 12, "Tilg, o Gott, die Lehren".
In the first phrase, we almost immediately have a descending minor sixth leap (Bb-D). This leap makes no contrapuntal sense in itself; it has a greater meaning due to the fact that the phrase, harmonically, goes I-IV-I. Melodically, this identifies three important degrees: the first (Bb), the fourth (Eb), and the fifth (F). The final F could have been part of a dominant chord; who knows, melodically what should matter to us is that the Bb-D interval is part of a melodic path that goes Bb-D-Eb-F, which is an ornamentation of a descending fourth interval, Bb-F. For those who sing, interpreting these melodic profiles is fundamental, sometimes, especially when there are "strange" intervals. At the bottom of the page, for example, a long and "tiring" modulation to F begins. This long modulation passes through strange melodic profiles, such as D3-G2-E2-F2-G2-C2. The E-natural MUST be thought of as the leading tone to F, and everything is an embellishment of the G, which in turn is part of the C chord. "Behind" this melodic line, it is as if there were two blocks: one that goes "Bb-C-D-Bb" and one that goes "E-F-G-F-E-D-C". Read this way, its intonation is immediate and very easy because it reveals the harmonic-melodic tensions. Moving forward, it gets even worse; there is a nice descending minor seventh, followed by an ascending sixth (C3-D2-Bb2). The final point of this sequence is A2 (it is not for nothing that it is part of the chord of the key not yet established but already "tonicized"—the F, on which the cadence occurs on the third beat). Without this beacon, all the intervals make no sense, they are difficult to intonate, and the text itself helps with this (the A is at the end of "lehren", at the end of the so-called textual unit).
The aria continues; perhaps I won't analyze it all, but reading its melodic line cannot be done without identifying functions (tonic, dominant, leading tone...).