In a way, I guess it's like sexual orientation. He just isn't attracted to anyone who isn't, as you put it, intrinsically connected to the fade.
I can't guess Solas' reasons, but considering how much he loves the Fade and all connected to it, and ancient elven culture, society, magic, values, etc... I think those things are really important to him. It might not even be like "sexual orientation" (which I suspect is a gross oversimplification, if you don't mind my bluntness), just about valuing those traits, valuing them in another, and wanting to share your life with someone who shares and values and experiences it with you right back.
It's always been difficult for me to understand sexual preferences and why some people are only attracted to females and not males etc. because I'm a bisexual (most would consider me pan if that helps in understanding me better). Gender and other things don't play into attraction for me, and it is almost usually the personality, or as Solas would put it, their "spirit."
If solas was a woman I would be totally crazy about him just the same. If he was a Qunari, dwarf, or who knows what, I still wouldn't care.
Um... huh? No offense, but in your last post you were upset about Solas not being into humans just because humans look physically similar to elves, which is itself based on looks rather than mind, spirit, or personality. "We're not super short and we don't have crazy horns or are twice his size. Humans are physically just like elves, just more curvy/robust, especially the females."
Personally, it's very likely that I'm asexual because I experience little to no physical attraction or desire at all, and what few trace amounts I do quickly fade away. I'm panromantic though, so I'm capable of forming emotional and romantic attachments to people of any gender, gender identity, or even level of conventional "attractiveness." To me, love and attraction is on a purely intellectual, spiritual, and emotional level. So in that way, I can understand Solas being attracted only to those he feels he can connect with on a spiritual, emotional, and intellectual level.
I guess Solas must believe that being elven effects the spirit directly. The implications of that ... are pretty huge, if you think about it.
I don't know. For all we know, he might be right.
When he talks about the Fade, he talks about being able to experience the thoughts and emotions of others as though he was there. He's witnessed countless battles from the perspectives of those who were there. Perhaps elven souls "feel" different from human or dwarf or qunari.
He also talks about a "collective consciousness," which might be related to those connected to the Fade being able to share and experience each other's thoughts and feelings. Like Cole, who's able to read the thoughts and emotions of those near him, like they're experiencing... a collective consciousness.
In fact, during Trespasser,
Perhaps Solas doesn't just "think" being elven affects the spirit directly; perhaps he knows from firsthand experience.
*shrug*