Life is almost 100% guaranteed. But think of all the things that had to happen for humans to come to be.
A planet at just the right distance to a stable star, rotating at a speed which allows it to be fairly uniform in temporary, making a relatively stable climate. Water, oxygen, etc.(not that all life requires the same chemistry) Mass extinction events that allowed us to be the apex species. Large planets in our solar system that help protect us from stray comets and asteroids. Earth's molten iron core, providing a magnetic field to protect us from solar winds, etc.
There's a lot that needs to happen to keep a species alive long enough to develop into an intelligent civilization.