The intention behind negative energy is the destruction of karma. Because the destruction of valued karma causes pain, this causes negative energy to manifest as pain in most cases. The intention behind positive energy is nourishment, which causes growth and strength in both beneficial karmic structures and limiting karmic structures. Sometimes it can cause tumors. Physical tumors or psychic tumors.
Negative energy feels like a "black hole" when the observer is passively funneling a smaller amount of positive energy at it. Since they cancel each other out it feels like will is becoming nothingness. This also causes 'festering wounds' and slow death, like a downward spiral.