You shouldn't. Crying means a baby is in distress for some reason, if a parent doesn't know what that distress is. Babies need to feel secure and safe, and they won't feel secure and safe if they are allowed to remain in distress. The longer they are upset, the more upset they tend to get.
In the first three years of life a child's brain is developing connections. The baby who feels safe and secure is more likely to develop the right kind of connections than the baby who is allowed to remain in distress. When parents are responsive to their baby the baby learns that someone will at least try to make him feel better when he's upset. That contributes to the baby's sense of security and trust of the most important person/people in his life.
When parents know how to make a child feel safe, secure, and responded to the child is less likely to cry in general.