I remind you that you are in "Physics Forums > Physics > Classical Physics".
In classical physics the cause of gravitation is mass.
Why mass causes gravitation is not answered by classical physics.
Why mass causes gravitation is not answered by classical physics.
Since answering questions like this must be strictly forbidden in the wrong forum, which section might one need to visit to have the aforementioned question answered...?
According to current generally-accepted understanding, gravity is "explained" by the curvature of spacetime in General Relativity, which is not considered to be part of "classical physics." See instead our "Special & General Relativity" forum.