Well, I completely agree with Unukalhai's definition. He says:
Simply put, synastry shows the interactions and dynamics between two individuals, but really has little to do with the relationship itself. That is the domain of the composite chart.
Composite charts show the dynamics and nature of the relationship itself once the people have actually started to merge at levels important to the inner self. This doesn't happen until a number of the person's defenses have been lifted, the relationship progressing from the 1st/7th axis to the 2nd/8th axis in astrological terms. The more merged the people become, the closer to functioning as one entity, the more important the composite chart is. When two people are only casually involved or not willing to "let the other in", the composite chart is all but meaningless, as there is no true balancing occurring.
I think synastry shows the effect the other person has on us...what we see in them (for each person), and stuff, right? And then the composite shows how the energies end up working out, so in my opinion, the composite shows the TRUTH about the relationship, because I'm supposing that even if from the synastry it seems capable of being quite romantic (and perhaps one person is..or hoped it would be romantic), how it ends up working in the end (shown by the composite) can be quite different!
This is an example of mine about how this happened to me personally with someone I had a big crush on for the longest time
"Hehe. Do I love him? Well...see it's just not that type of relationship. I see it more as a very caring friendship than anything. I do love him. But not in that way. I am really attracted to him....used to be so much that I was SORT of infatuated with him lollllz (ACtually), sigh. But, the infatuation always goes down once you get to know them and stop idealizing so much, right? That always happens when I have a crush. At first, I idealize him and think of all the ways he probably is and the things he likes and dislikes. But of course, that's just me projecting onto them what I WANT for them to be like, right? lol. But then you get to know them and you learn that you were often way off about the way they *probably* were, and that it's ok for them to be different, because that's them, and you learn that you can accept them for who they are.
For me with him, this process took a lot longer than with, say, another guy I might get a crush on. It's because the emotional intensity that was aroused whenever i would see him, and the very very sexual/erotic element, too. I thought he was really hot! and I never really felt that way about a guy...I mean, good looking, sure...but never really Hot. So, yeah, it took a while to realize that he is a lot more than what I *wanted* or *wished* or *imagined* he would be, get me? lol yeah, lots of words. But yeah, after the infatuation was over, now I just see him as really good friend, pretty much. Someone who I can trust, and talk to. And well, I DO love him. But, not in that way. It's different. I've realized our relationship is actually much more platonic than i used to think it would be. We're more like brother/sister, and less like boyfriend/girlfriend lol. It just feels different."
At first I thought he was the perfect guy for me. I was very romantically attracted to him (our venuses are conjunct, my venus trines his uranus, my MARS conjuncts his Venus, my pluto squares his venus, my mars semi-squares his sun, his sun opposes my uranus, my moon trines his mars, etc). I thought we could build the perfect relationship and saw us being very romantic with each other in the future. These are all things that being around him wihtout having opene up, like unkalhai said, made me feel. That was because of our syynastry. But THEN, after I got to know him, I felt the relationshiop felt much more appropriate being platonic, idealistic, but not romantic in the love boyfriend/girlfriend sense. The Composite chart shows a completely platonic connection. Sure, idealistic, but platonic (meaning only friends, not lovers). This now feels very right. But before having thought about all of this, I was still stuck with the mentality of my mars conjunct his venus for example, "No, he is so attractive to me, I just can't let that go. I want to chase him and be romantic with him, and flirt with him." But now that I know him better, when I see him, it just doesn't feel right to do that. It feels like we are too much brother/sister to do that (even though we are not, or even related). But before having gotten to know him, I probably would've still been stuck in how he affected me and not in how the relationship felt in general...because there was no relationship. Lol.
-DB