Your natural teeth have several degrees of sensitivity and flexibility. The tooth itself is flexible. Though it may not seem like it, your teeth are somewhat squishy, and when you bite and chew, they compress. There’s also a nerve inside the tooth that senses how hard you’re biting down.
Add this to the periodontal ligament and you’ve got a good idea if you’re biting down too hard with natural teeth.
With dental implants, though, there isn’t much sensation. Only the bones have some sense of how hard you’re biting down, and the bones aren’t really designed to give that information, so it can take a little bit of time to learn to not bite down too hard.