Realms of unhappy spirits / hungry ghosts also known as “Pretha loka” is a rebirth based on strong possessiveness and desire which were cultivated in a previous life or lives.  They are constantly extremely hungry and thirsty, but they cannot satisfy these needs.

"When one is reborn as a peta (a hungry ghost) having done evil deeds which violate moral laws,

one will never get at least phlegm and snot (as drink). One's throat will remain parched even if one falls into the heavenly river.

It is impossible then to do any merit, even if one wishes to"

{From - Lowada Sangarawa (towards a better world); a Sinhalese poem of the 15th century by Ven. Vĩdãgama Maitreya Tero & translated by Bhikku K. Nãnananda}