Should we pray to God for something we need just once, believing with great faith that he will answer us? I think not. I believe that the only time you don't need to ask him repeatedly is when you abandon yourself to him and ash him to do whatever he wishes with you. Otherwise, for other things you feel compelled to pray for, such as the conversion of a loved one, you need to pray unceasingly. It's not that you lack trust in God. On the contrary, you want to show how much you need and rely upon him. Suppose you have two children. One comes to you and asks for something s/he needs just once and the other one asks you for what s/he needs day and night nonstop, which child to whom you are more likely to give in? I think the answer is clear. Since God is our loving Father, the more we ask him, the more endearing we are to him. Again, I am thinking common sense here. As a matter of fact, I feel closer to the Lord each time I go to him with the same petition. Another reason, since the world sorely needs our prayers, praying unceasingly to God every chance we get for the world can help fulfill that need.