The tooth has 3 main layers; the outer white enamel layer, the middle yellow dentine layer and the inner layer, the nerve (i.e. pulp). The enamel layer is similar to your nails in the way that it does not have any nerve endings and hence no pain when decay goes through this layer. The nerve endings start in the pulp and go into the dentine layer , so when decay starts entering this layer, some people may feel something like sensitivity, others don’t. By the time there is significant toothache or pain, more often than not it has reached the nerve.
Hence it is very common to have decay with no pain, and this is why it is essential to always get checked.