Your knuckles don't need to be cracked all the time. It is not necessarily a physiological need as much as it can feel good and be a habitual thing that you do. There is nothing wrong with it and it will not give you arthritis. The tightening of your joints may occur as you use your hands more, especially with a mouse or for typing. These repetitive motions may make your knuckles tight, so that when you mobilize or 'crack' them it loosens the joint, release pressure (the crack) and feels good.