Here's why:

* Extreme Density: Black holes form when massive stars collapse under their own gravity. This collapse squeezes an enormous amount of matter into an incredibly tiny space, resulting in extreme density.

* Escape Velocity: The gravitational pull of a black hole is so intense that the escape velocity (the speed needed to escape its gravitational pull) exceeds the speed of light. This means that nothing, not even light, can escape from within a black hole's event horizon.

* Singularity: At the heart of a black hole lies a singularity, a point of infinite density where the laws of physics as we know them break down. The gravitational pull at the singularity is considered to be infinite.

Important Note: While we say gravity is "infinite" inside a black hole, this is more of a theoretical concept. Our current understanding of physics breaks down at the singularity, so we can't truly describe the nature of gravity there.