How does not knowing the details of the Lord's return motivate you to live righteously?
Ideally, we would not be influenced by knowing or not knowing the details of when and how Jesus Christ will return to Earth. However, given our frail human nature, it does matter. Because we do not know the details, and we know that Jesus might return “like a thief in the night,” we must live our lives so that we will always be ready to be judged. This motivates us to live righteously.
If we know the day and the hour when Jesus is to return, we might think that we can be lax in our behaviors. We might think that we do not have to be as righteous as possible because there will always be time to repent in the future. This is surely wrong since God knows if we are trying to manipulate him in that way, but it is how we tend to think. However, since we do not know when Jesus will return (or when we might die, for that matter), we have to live life righteously. We have to fear that Jesus could return at any moment and we had to fear what would happen to us if he were to return when we were not prepared.
We really ought to live righteously simply because we know that is what God wants from us. However, we are weak and we sometimes need to be motivated to do what is right. Our uncertainty can give us such motivation.