Pars pro toto means: a part (taken) for the whole. We think of one trait or one particularity of something – or someone – to represent its entirety – or the whole person.
It often happens in the early stages of a romantic relation. One is attracted to beautiful eyes, a finely chiseled mouth, or some other physical detail (or perhaps something that was said — but we usually don’t work that way) and then boom ! We fall in love, believing the whole person is as wonderful as that very first smile that founds its way to your heart.
Sadly, it’s the most stupid thing in life. Or the most amazing one. I am not sure. Perhaps it is both.