Babies have an immature digestive system and pooping involves the coordination of voluntary and involuntary muscles. Does you baby have hard, pellet-like stools? If so the baby is constipated and you should talk with your doctor about this. If the stools are soft then you can try massaging the tummy and/or putting the baby on the back and pulling the knees into the baby's tummy. This changes the angle of the rectum and helps pass gas as well.
Re: Why the straining?
good luck!