When an individual is so pleased with another individual that out of natural, deep affection and love, the individual wishes benefit for another individual, then that wish is known as a blessing.
Likewise, when an individual is so displeased with another individual that out of natural, deep anger and sorrow, the individual wishes harm for the another individual, then that wish is known as a curse.
Both blessing and curse arise out of extremely felt deep emotions and are thus highly potent and effective.
The Mahabharata has innumerable instances, where blessings and curses are either given or received.
Mahabhisha and Ganga were both cursed in heaven to lead sorrowful lives on earth as Shantanu and Ganga.
Bhishma was blessed by Shantanu to die whenever he wished to die.
Bhishma was also cursed by Amba to receive death because of her.
Rishi Durvasa blessed Kunti with a boon whereby she could summon any god and obtain a son.
Pandu was cursed to die whenever he tried to make sexual advances.
God Indra blessed Karna with the heavenly weapon Indrastra.
Gandhari cursed Krishna that he would not be able to do anything when his entire Yadava clan would fight and die in front of his eyes.
And so it happened.
There are also many other short stories in the Mahabharata, where curses and blessings occur.
All these incidents point towards the fact that when a person feels deep emotions, and then wishes for something, that wish invariably and ultimately fructifies.
All the forces of the universe act together to fulfill that blessing or curse, as the case may be.