Think about it from the perspective of his cumulative actions. They have been more genuine, honest, and clear than his meager vocabulary and clumsy body language would ever communicate. Here is a pathetic, lonely traveler who is not just capable, but willing to put up with Horo.
Let's face it: most of us would have cut our losses long ago. Horo rarely gives genuine praise, and usually follows it up with some snarky or savage putdown. Even before this story arc, Lawrence has proven that he is a loyal companion that did not want to let Horo down. Heck, he almost had a heartattack when he struck her hand that one episode. She is honestly lucky to have found him.
Poor Horo, though! Despite all her belittling, he can still scheme with the best of them.. even without her help. He stands to make a mint from the pyrite scheme (which he even deduced on his own, in spite of Horo). Even if she leaves him, he'll profit from Amati's contract. He's even had the courage and bravado to look Amati straight in eye and challenge him in turn, despite being so obviously desperate.
And yet, despite the fact that his dream of opening a shop is probably within his grasp if he does all of this, he gives it all away in an instant. He doesn't care about the profit, nor even about revenge against Amati. No, it's simply because he absolutely cannot lose his most precious cargo.
Clearly Lawrence doesn't see Horo's value in terms of the profits she can bring him. He values her above profit period. How could anyone, pagan deity or not, not feel ashamed if a hardened merchant spoke such blasphemy for her sake?
Adding injury to shame, his "stupid question" clearly indicates that he wants to take their relationship beyond simple companionship. If Horo does want more from Lawrence, she clearly has the opportunity. It's telling that she can only throw those words back at him, and pretend to be angry. She really is asking too much of poor Lawrence.
It's pretty evident that Horo wasn't actually shamed by Deanna, but rather by Lawrence. I even get the impression that Deanna encouraging Lawrence to "do his best" was really calculated more to wound Horo than to inspire Lawrence. It somehow feels good to see Horo get a taste of her own medicine in this way
Deanna, Mark, and especially Lanto are the real heroes here. Horo owes them even more than Lawrence does. If Horo left him, Lawrence could probably cry himself to sleep but then pull up his bootstraps and become an even more determined merchant.
On the other hand Horo might be completely screwed. Even if Amati let her go, it's unlikely that she'd ever run across another gentle spirit like Lawrence. She would have to travel all alone to her hometown, which may not even exist anymore. That certainly would not be pleasant without a compassionate companion willing to go out of his way to take her there.