YouTube captions help to make the service accessible to all types of users. With that in mind, if you want to add auto-captioning (based on speech) to every video you load, you can. Or if you want to disable all types of captioning, you can do that too. Follow these quick steps to find out how to get to these settings:
Log in to YouTube so you can access your personal settings.
Click your username in the top right to open a small menu, and then choose Settings.
Choose "Playback Setup" from the next menu on the left.
"Captions" is the second section here; adjust the settings to your liking.
Don't forget to hit the "Save Changes" button at the bottom!
You can always change these settings at a later date, but it's nice to have control over what is going to be shown to you by default. Now go enjoy yourself and watch some videos!