I've read something about fluid interface and know that liquids tend to reduce their surface but my question is that why do they tend to reduce their surface?
Surface tension is the result of a mismatch in interaction energies between neighbouring fluid particles.
Imagine a water molecule in the bulk of the fluid; everywhere around it the molecules are similar to it and it will therefore experience the same interaction everywhere around it. The net interaction force is therefore zero.
Now imagine a water molecule at the water-air interface. One one side it sees water molecules, on the other side it sees air molecules. A water molecule interacts differently with a water molecule than with an air molecule. Therefore, the net interaction force is not zero anymore as there is stronger interaction with water than with air.
Nature in general doesn't like mismatched energies and will do everything to minimize the net energy mismatch. It does this by pulling water molecules inward in such a way that the total surface area is reduced.