Dvr

Can someone tell me why things that I have had on my DVR for a while mysteriously disappear? I don't understand the rationalization of having a DVR so that you can record things that you enjoy watching and then Xfinity in their infinite wisdom besides to delete things that have been on the DVR for over a year or two. I don't find that to be fair, why is it decided by someone else what my wife and I are allowed to watch? We recorded it for a reason and then it disappears? The worst was I had the godfather saga in its true Canon form which you cannot get anymore and that was erased years ago without my knowledge or approval. These things just don't seem to be fair and I wish that Xfinity would stop doing this