There are many reasons why succulents may die. My experience has been that when they die it is usually due to overwatering. If you are growing them indoors, the easiest way to know if your plant is getting too much water is by checking its top leaves. The top leaves should be shiny and glossy and not droopy or wilted. If they look like this, then the plant needs more water. The other thing you can do is check the soil. You can see if the soil has gotten wet when you press down on the top leaves. The soil should feel slightly damp. If it feels wet all the time, then the plant needs less water. If the soil feels dry, then you need to add some water. If you are growing your plants outside, then you can look at the soil to see if it is getting too dry or too wet.