Water attracts water: water molecules are attracted to each other. Much like magnets, water molecules have positive and negative “ends” that stick to each other due to a phenomenon called cohesion. In the same way Spirit attracts Spirit, there is an attraction, a drawing of the Holy spirit in us towards the Holy Spirit in heaven and vice-versa.
The bigger the build up of water the more pressure there is for its release, there are things that hold back the water of God’s Spirit in us like dams on a river but when the pressure rises sufficiently (i.e. we are filled to overflow by the Spirit of God) then the dam breaks and the Holy Spirit flows freely out of our lives and we get revival.
When the pressure of the Spirit of God is greater than the resistance to His will within us then we surrender to His will and His kingdom can be manifest in our lives. Deep calls to deep so His love can be manifest in us and through us with authority, power and dominion.