It feels like I'm saying it a lot but...Solas. It just feels so incredibly deep and meaningful. And I'm not even usually a fan of star-crossed lovers-type situations. But it's written so subtly and so well. It really makes you feel for the character, which adds to the impact of the ending.
Heh, I can see how that would be attractive.
Me personally I don't like how there's that huge part of him who'd rather stay out of romance with you for numerous reasons, no matter how much he admires you. 
Hmmm, I think that simplifies things a bit. Solas is a man who's pulled between very conflicting desires. He is driven by guilt. He wishes he could forget about the past and live in the moment but he can't allow himself to forget about what his actions have caused, because that would be wrong.
At the same time, though, he is also incredibly lonely. He's spent thousands of years in seclusion, with only spirits for company. In this new world, he's a stranger in a strange land. Everyone he'd ever known is either locked away or long-dead. It's also hinted that he was betrayed in the past by one of his closest friends/allies. And ever since he woke up, everyone in this new world that he's tried to discuss things with has treated him like he's insane, especially the Dalish. So he finds it very difficult to trust or rely on the Inquisition, especially given how he realizes he's deceiving them all. And yet, he still so obviously cares for them. Even though he tries to keep himself aloof, he paradoxically yearns to be connected with those around him.
It's the same thing for the Inquisitor, but magnified a hundred-fold. The Inquisitor is always going to mean something to Solas, regardless of whether they romance or even like him. They're the one who ended up with his power, and the one who's actually going out and fixing his mistake. And if the Inquisitor goes about it in a thoughtful way, Solas sees someone in this new world using his power to actually make things better, something he'd all but thought impossible. Furthermore, by helping his friend, the Inquisitor demonstrates that they're someone that Solas can personally trust and rely upon.
And if the Inquisitor romances him? Not only is this someone worthy of admiration, who he can trust, but someone who loves him and is offering him the companionship that he's yearned for for thousands of years. Is it any wonder he falls so hard?
He's also painfully aware of how unethical it is to lie to them while in a relationship, which is why he's always so hesitant. In the final romance scene, he's finally decided to tell the Inquisitor who he really is, because she deserves the truth (confirmed by his writer). He takes her to a nice place, works himself up to do it, and then...chickens out at the last moment and tells her about the vallaslin instead. So then he's left with a choice. If he isn't able to tell her, then he either has to end it or keep lying to her. So he ends it.
It's not a simple matter of Solas not wanting to be with the Inquisitor enough to tell her who he is. Probably more than anything else, he wants to stay with her. After all, his greatest fear is "dying alone". He wants to be able to tell her the truth. He tries. And yet he isn't able to do it. Not because it may mess up his plans, but for the very human reason that he's scared and not used to letting anyone past his walls.