Megara sold her soul to Hades, in order to revive her boyfriend who had died. Hades agreed on the condition that she serve him forever. However, shortly after her lover was revived, he fell in love with someone else and left Meg still locked in servitude to Hades, abandoning her even though she had sold her soul because of him. This leaves Megara feeling heartbroken and determined to never fall in love again.
(Source: bleedingstrawberry)