Why do I always have to crack my neck? Is this really bad?

No it is muscular. This is ok and you are doing your own spinal manipulation like cracking your knuckles. You are most likely "cracking" your facet joints (the small joints of the spine-2 at each level) which stretches their joint coverings or capsules which causes a reflex relaxation of the surrounding muscles. There is no harm in you doing this.