Having more than 1 kid.

Honest question. I noticed that almost every single person I know that has a child, has another within the next two/three years after the first. Why is this? Do you get baby fever again or just so they have a sibling close in age? I always fear that having a baby and a toddler at the same time would be too difficult but I notice that even people who didn't want children before, follow this pattern. Just interested to hear your thoughts and stories! I hope I made sense.