Tolkien used √SI as the basis for “near demonstratives” like “here” and “now” from very early in his writings on Elvish. The Qenya Lexicon of the 1910s had two competing roots ᴱ√HYA “this by us” with derivatives like ᴱQ. hyá “here by us” (QL/41) and ᴱ√KI “this by me” with derivative ᴱQ. tyá (< ᴱ✶kı̯-ā) “now” (QL/41, 49). Indications of the latter can be seen words in the contemporaneous Gnomish Lexicon such as G. cî {“now” >>} “here” and G. cîrin “present (place or time), modern” [gloss deleted] (GL/26). However, Tolkien also introduced a new root ᴱ√si(n) “this here by me” with derivatives like G. sî “here” and G. sith “hither” (GL/68). Revisions of Gnomish ci- word glosses indicate Tolkien was vacillating on which forms were temporal and which were spatial.
In The Etymologies Tolkien gave the root ᴹ√SI “this, here, now” with derivatives like ᴹQ. sí or sin “now” and ᴹQ. sinya/N. sein “new” (Ety/SI). The root √SI was mentioned a couple times in Tolkien’s later writings, usually glossed “this” (PE17/67; VT48/25; VT49/18) and in one place with the variant √SIN (PE17/67). This root was not entirely without competition in Tolkien’s later notes, however: in one place he gave primitive ✶khĭn- as the possible basis for Q. hí “here” and S. hí “now” in 1968 notes on demonstratives, though it appeared beside primitive ✶si- forms (VT49/34 note #21).
This root was the basis for Elvish “love” words for all of Tolkien’s life. The root first appeared as ᴱ√MELE “love” in the Qenya Lexicon of the 1910s with derivatives like ᴱQ. mel- “to love”, ᴱQ. meles(se) “love”, and ᴱQ. melin “dear, beloved” (QL/60). In the contemporaneous Gnomish Lexicon it had similar derivatives like G. mel- “love” and G. melon “dear, beloved” (GL/57).
In The Etymologies of the 1930s Tolkien specified that ᴹ√MEL meant “love (as friend)”, and for the first time it included the derivative N. mellon “friend” (Ety/MEL); Gnomish “friend” words from the 1910s were mostly based on G. ged (GL/38). However, the same entry included ᴹQ. melindo/ᴹQ. melisse “lover” (male and female), so it seems even in the 1930s it could refer to romantic love (Ety/MEL). The root continued to appear in Tolkien’s later writings associated with “love” (PE18/46, 96; PE17/41; VT39/10).
In notes from 1959, Tolkien elaborated on the precise sense of √MEL and its role in romantic and non-romantic love:
> Love, which Men might call “friendship” (but for the greater strength and warmth and permanency with which it was felt by the Quendi) was represented by √mel. This was primarily a motion or inclination of the fëa [“spirit”], and therefore could occur between persons of the same sex or different sexes. It included no sexual or procreative desire, though naturally in Incarnates the difference of sex altered the emotion, since “sex” is held by the Eldar to belong also to the fëa and not solely to the hröa [“body”], and is therefore not wholly included in procreation ... The “desire” for marriage and bodily union was represented by √yer; but this never in the uncorrupted occurred without “love” √mel, nor without the desire for children. This element was therefore seldom used except to describe occasions of its dominance in the process of courting and marriage. The feelings of lovers desiring marriage, and of husband and wife, were usually described by √mel. This “love” remained, of course, permanent after the satisfaction of √yer in the “Time of the Children”; but was strengthened by this satisfaction and the memory of it to a normally unbreakable bond (NM/20).
Thus √MEL was close in sense to Greek “philia”, used of friendship, whereas √YER was used of “eros” or sexual desire. But in Elvish thinking, √MEL was essential for romantic love, and √YER only arose from that. Furthermore, √YER was not the most important element in the love between romantic partners, as the period of procreation and child-rearing took up a relatively small portion of Elvish lives. It was the more enduring feeling of friendship between lovers that really mattered, and thus √MEL was used of both non-romantic and romantic love, though it had not particular sexual connotation.