There will be a surprise party for Mark tonight. He loves surprise parties, because he loves the surprise—but he'll feel terrible if the surprise is spoiled, and not want to go. He has a reason to go to the party—the surprise—; but if he knew or even believed what that reason was, he would have a reason not to go. It looks like in this case there can be really good reasons for someone without the person being aware of what those reasons are (and in this case, it wouldn't even be a reason to go anymore if he was aware of it). Now presumably this reason isn't motivating Mark to go to the party. But it is a fact that counts in favour of Mark's going to the party even without motivating him to do it.