Why is each pregnancy so different?

Granted this is only the second time that I've been pregnant, but it is vastly different than the first time around. With Xander I had pretty much no symptoms of pregnancy until towards the end. In fact, I didn't even know that I was pregnant until the second trimester had already begun. This time I knew within at least a month. My appetite had changed, I had morning sickness and fatigue (still have fatigue), and my hormones are going crazy. I get frustrated easily with everything and nearly everyone and get emotional at a tv commercial! I crave bad food and just want to do nothing all day long. What is up with that?!