Answer: Wanting or need attention.
Explanation: Toddlers are freshly out of babyhood, so not many of them can say what their needs or wants are other than pointing or crying so the parents or others will have to guess. In the event of a new sibling being born, toddlers will age regress because they want just as much of attention as their baby brother or sister. So they will act out in the wrong ways to have their parents or others see them, so if you feed into their bad behavior, they will continue to do just that.