Life is a tough thing which we have to bear with. It gives us pleasure and pain. Pleasues are temporary but, pains are much enduring. They don't easily vanish. I think the reason behind it may be the way our mind works. Our mind gets sence of pleasure due to releasing of chemicals called neurotransmitters. But, this pleasure is only lasts longer till those chemicals work. On the other hand pain have permanent effect. Suppose one have lost one's hand in an accident, then that person would have to remain without hand for whole life. So, pleasure cannot bring you something extra but, pain can snatch something important from you. That's why pain is mightier than the pleasure. Pleasure generates short memory while pain has everlasting effect.
Love you @erica005