At the risk of stating the obvious, having a baby is a massive commitment. It's also a lifetime's responsibility. You get a little help for the first day or so while you're still in the hospital, but then it's over to you to keep this baby alive for the rest of its childhood and adolescence. If you've never done that before, you panic at the slightest unusual noise. Worse still, you panic when there's a lack of noise!
Sometimes, a quiet baby is just sleeping. That won't stop you from worrying about it, though. When you worry, you go in to check. When you go in to check, you're probably going to wake your sleeping baby up and have a grumpy, tired, crying monster on your hands. After a while, you have to persuade yourself somehow to take a step back, and trust that your newborn isn't going to turn into dust the moment you turn your eyes away.