It's normal for them to cry for a while until they get used to being left alone. My little one would cry and howl like a baby coyote
when I would leave for work in the morning. This went on for a while until he got used to the fact that "Mommy has to go to work and I have to stay here in my safe, comfortable X-pen until she comes home." For the first several weeks he was also frantic when I would come home and take him out of the pen -- crying and kissing my face frantically. THEN ONE DAY ... he was quiet when I left in the morning and quiet when I came home, patiently waiting for me to take him out of the pen. Now he's just an absolute love when he's got to be alone. I think it's just part of growing up.