I Am Fascinated By Human Behavior

can't understand why do people use others as their punching bag.they're too coward to cope with others & themselves & want to release their stress & the funny is they always do this to their beloved.simply they're taking advantage of love & exploit love.it's ridiculous but i can't understand why do other tolerate them?is love needs all this?or love isn't sufficient to cope with tension?or they're too selfish to feel love?or just sadistic