I feel that it happens for several reasons. First, you evolve with everyone of your production, you learn new techniques and get better. Second you probably heard the track several hundred times working on it so you get a bit fedup by the melody. All of these make you not specially like previous track, it happens to me all the time. Some I did in the past, i thought it could totally be ok for labels, and when I listen back I am happy I didn't sent it! haha