Both of my Labs will go into their own crate numerous times throughout the day to nap or just rest. In the evening they will also put themselves to bed at around 9:00 PM or thereabouts. All I have to do is remove their collars, give them their good-night snack and shut the door.
If your dog is going in on his own during the day it's, most likely, because he feels comfortable and safe there. Does he appear to be relaxed while in the crate? You should be able to tell.
Labmomma