a biological explanation

It is hard to forget someone because:

after so many years of thinking about the particular person, countless neural connections has been made,

particular neurons are more susceptible to activation when exposed to neurotransmitters emitted by neighbouring neurons,

which in turn are activated by sensory inputs from the environment, like a rose or a piece of melody;

and long term association by the mind, when exposed to such poignant environmental stimuli,

coupled with the current activation of certain fond memories of a certain person (which was probably all the time),

led the brain to form neural pathways, hardwired by the ease of certain activation of neurons by certain stimuli,

like a memory node, the crossroads of all thoughts,

thus she is unforgettable.

But time will cause all to fade,

as old neural pathways are replaced by new, old neurons dying off and replaced by new,

shifting the center from one to another;

Unless, that is, of course, you are still thinking of her.