Why isn't electrostatic attraction cancelled out by electrostatic repulsion?

Question

Van der Waals interactions involve two molecules, when they are very close together, exhibiting attraction to each other as a result of instantaneous and very brief shifts in their polarities (e.g. consider molecule A on the right side of molecule B; the electron cloud briefly shifts to create a positive polar charge on the left of A and a negative charge on the right of B resulting in attraction).

However, why aren't these interactions also instantaneously cancelled out by repulsions that arise for the same reason? So, for one moment A is positive and B is negative, but the next moment A is positive and B is also positive and they repulse with the same strength that they interacted. What gives?

Answer 1

It is not canceled out because it is advantageous from an energy perspective to have the electron clouds shifted to cause attraction. This means, on average, the molecules will be in an attractive configuration more often than a repulsive configuration.

Answer 2

i dont get what do you mean by at the next moment A is positive.

if A is more electronegative than B then electron cloud in the bond will be closer to A compared to B, so A is negative and B is positive and that will always be the case.

if you think there is a possibility of A and B to be positive how do you think a neutral molecule randomly attained a positive charge?( as both have positive charge the net charge looks like positive)