Permit me, please, to explain this from a psychological standpoint.
When one is said to love another, it is for one or both of two reasons: Concupiscence and Projection. Concupiscence has more to do with the subject's physical appearance and ability (and standing), though not much to do with the actual person, or perhaps as we say, their soul. Projection is unconsciously projecting one's own inferior traits on another person (in our case, at least). Jung termed this projecting his anima (feminine, which a male projects on a female or on children) and animus (masculine, which a female projects on many males). Together they make the the syzygy. <PERSON> explains somewhat similarly, though using his four intelligences instead.
The idea is, since we are strong in one but weak in another (or, according to <PERSON>, because it is unconscious) we project in on another person who is strong (or conscious) of those traits, and we develop ourselves through the other person. Being the other person is the subject of part our own self, we come very close to the other person, seeing them as part of ourselves. Whereas by concupiscence almost anyone will do and we feel in control of it, by projection, few people become the subject of our projections and we do not feel in control of it. Strong projections, much like strong desire, hurts when it is not satisfied. Unlike desire, it takes another person to willingly satisfy it.
When two people project on each other, they are drawn together unconsciously. Being they do not know why (projections are by definition unconscious (there are many levels of awareness)), they feel "drawn" to each other and "not in control" of it. This "unknown force" is often given a name, such as fate or love, the latter more common in Xian circles where love is held in such high regard.
The Xian aspect is important. 674
Xian belief is love is a virtue but concupiscence is (at least related to) sin. Hence, people may want to make sure their love is "pure" or "true", that is, free from being based on concupiscence. With this, the love is considered special. Paradoxically, this does not preclude concupiscence from the relationship, just from being a basis of it.
<PERSON> was (and possibly still is) an immature person who is scared of anything or anyone that he cannot control. When he became <PERSON>, he had more power than anyone else (at least in the first season) meaning he was now in control, or at least, noone else could control him. When <PERSON> came along, everything changed, because he projected his own frailties onto her and therefore loved her as he loved himself. That's why he won't kill her, why he let her go free, why he wants her to think he trusts her, and why he is, at times, at her mercy. Being out of control, he calls it true love, and would have been "cured" by it, until he asserted control and kept her at a distance. For reference to <PERSON>, <PERSON> as the Dark One is an INTJ, and <PERSON> is an ENFP. The NT and NF are drawn to one another.
Similarly, <PERSON> can be <PERSON>'s true love, if they would stop asserting control. They do have strong affect on each other and even act "out of character" accordingly.
Can one have multiple true loves?
Yes. People can project on many people. 674