Question

why a liquid seeks its own level?

Answer 1

This happens because at the bottom,i.e at the long tube, the pressure should be the same. ... Hence to keep the pressure same, they attain the same height. So you may force the liquid into whatever shape of vessel you wish, it will always seek to attain it's own level.

Answer 2

Density is the same for all of them and so is the acceleration due to gravity. Hence to keep the pressure same, they attain the same height. So you may force the liquid into whatever shape of vessel you wish, it will always seek to attain it's own level.