Say yes to everything and your schedule gets run by whoever asks first.

Agreeing by default hands your week to whoever asked first. Requests do not arrive sorted by importance, they simply arrive, so the earliest and loudest win and your own plans get whatever is left over. That is how people end up busy and behind on the thing they actually cared about. Choosing is not selfish, it is the only way your priorities get hours.

Before you answer the next request, look at the week and find what it would displace. If you cannot name what gets dropped, you are not ready to say yes.