You get self-cracks if the skin is so dry and thick that the outer layer of skin bursts. You can treat and prevent self-cracking yourself.
Hardening and self-cracking can be troublesome. Here you will find tips on what you can do to relieve the hassle.
You may get a hardening if you use shoes that press against the foot. Then the skin becomes hard and thick. You can then get a cornucopia if the hardening is strained or worn for a long time. Then the hardness is pressed into the skin like a thorn and it can hurt a lot. You can get rid of the hassle but it can take time.