You can enable or disable embedding of your video on external sites by going through the following steps:

1. Sign into your YouTube Account.
2. Click your username located in the top right corner of any page. Select My Videos from the drop down tab that will appear.
3. Click the Edit on the right of the video in question.
4. Scroll down. Under Broadcasting and Sharing Options, click Embedding.
5. There you will be able to select the radio button next to Yes, external sites may embed and play this video
   or
   No, external sites may NOT embed and play this video depending on which you prefer.
6. Remeber to click Save Changes at the bottom of the page or else your edits will be lost.

You can change this option for your videos anytime you want.

You will have to do this for every video you want to embed!