I really think that the draw to Solas for me was in his loneliness and the eccentricity solitude fosters. He says himself that he has basically no friends who are not spirits. In Veilguard this is only reinforced when he says it took him centuries to build a rapport with members of the rebellion. Sometimes it just hits me how lonely he was.
I also think it's interesting that, if the Inquisitor romances Blackwall, Solas tells him that he's glad he's found some happiness despite everything. And when Blackwall asks him if he has found someone, Solas says 'no, I find my peace elsewhere'... he's really not all that romantically inclined. One of the reasons I hate most solavellan fic is that people tend to write him as this super experienced fuckboy when the text tells us over and over again that he doesn't form bonds with living people very easily. I get that some people find the idea of being with someone who has thousands of years of experience sexy, but the text tells us repeatedly that he isn't especially invested in relationships with non-spirits, who are non-sexual, so he probably isn't as experienced as people think. I won't get into it too much bc I've already spoken about it, but he never properly learned to bond as a person and not as a spirit. So prior to DAI, he simply doesn't, and we can infer--especially given that he's described as never having been in love before--that he hasn't pursued romance a great deal in his life.
This is probably fed by my own bullshit (it's definitely fed by my own bullshit), but I think I had a hard time figuring out how to romance him bc they would only allow you to romance him with a female elf due to time constraints. This is despite him saying repeatedly that he loves Lavellan's spirit. When you're trans or nonbinary, it's hard to explain how exciting that is; to have characters who say explicitly that they do not define others by their bodies. It's very cool on its own, but for nb players in particular it's especially important. If you struggle with how you're perceived physically, it's nice to be able to make a character who is loved for who they actually are and not for what they look like. To have someone who sees them for who they are and not for what others want them to be. It's something people like me crave and I think this is why I'm so pro-queer Solas, pro-Solas doesn't give a shit about how on fleek your Lavellan's eyebrows are, etc. The physical doesn't factor in to why he loves. He simply loves others for their character, and although he often lets his first impressions colour his views of entire people, he is still willing to listen and learn if you give him a chance to be Wisdom and not Pride. We literally see him begin to question his own convictions re: the Dalish during the balcony scene. Contrary to popular fandom belief, he actually admits to being wrong all the time.
Idk man I just really like having a Lavellan that is an outsider all their life and isn't understood even by their own Clan, only to find her kindred spirit in Fen'harel, the adversary of her people. I'm obsessed with the fact that Solas fell in love for the first time with a mortal, who exists because of his mistake, who he loves enough to throw away his plans and only doesn't follow through because of his guilt. I need to write more fic jsdkdfhkjsg