I had the same issue, and then I did a bit of reading. I'm sure some of it is slightly inaccurate, but the gist of it is what matters... Someone can correct ;)

When you post a video to Dtube, the video is stored on someone's node. I assume that's like Bitcoin Core Wallet, except it has its own associated coin. Nodes temporarily hold files, so what you've uploaded won't stay there forever unless the following:

1- someone pins your file on their node and it stays put.
2- more people constantly watch your video, allowing more copies of the video to be spawned/fetched/stored on more nodes. And each time a new copy of the video comes to exist, the expiration for your video gets pushed back, keeping it alive longer.

Me thinks I'd better run my own node and pin my own videos...