I think the most disheartening reason would be that the person
you love can't forget her/his previous relationship and actually
still loves him/her after being with you for so long.
It can actually literally crushed your heart. Imagine this, while
this person you love, which you thought loves you as well, is
actually together with you physically, doing things which you
thought is what he or she enjoys doing with you, while his/her
heart is actually not with you. Worse of all, you didn't know
anything until a day when he/she tells you out of the sudden, just
when you think your love with him/her is at its peak.
WTF (what the fish)
