It's not based entirely on him liking Lavellan, I won't deny that it plays a part. And approval has nothing to do with it. You only need high enough approval to start the romance, it ends the same way for everyone. Depending on how Lavellan chooses to react, Solas may have a different line to say once Corypheus is defeated and he's pouting over the broken remains of the Orb. Paraphrasing here: "No matter what happens, I want you to know that what we had was real" OR if Lavellan got angry and shoved him "You were right to be angry with me. I hope in time you will see why." Not exact quotes, but that's the gist of what he says.
You still have a chance to break things off when he takes you to the cave in Crestwood (what I lovingly call the fire swamp). When Solas takes Lavellan to the fire swamp, he has no thought about bringing the relationship to a full stop. He has no idea he is going to panic and back out badly. Solas took her there to tell her everything. About himself, Corypheus, the orb. All of it. He was going to give up on his plans, whatever they were, to stay with her. We know this by word of Weekes. It really is at the very last second he realizes he can't bring himself to tell her.
"I was trying to determine some way to show what you mean to me. For now the best I can offer is...the truth."
Those last two words there, slight hesitation before he says "the truth", that is when he comes short and decides to tell her about the Vallaslin instead. After that he realizes he cares about Lavellan so much that he can't continue keeping the truth from her, and he cannot drop whatever plans he has, so he backs out. Badly. I hope that makes sense.
You can talk to him afterwards and get him to promise to tell Lavellan what the Hell that was all about.