Why some people aren't necessarily attractive at first glance but after some time of you getting used to them you find them appealing?

For example if you're friends with a person and at first glance their appearance didn't do much to you but over the time you started getting used to their image and they start become appealing to you?

Why some people aren't necessarily attractive at first glance but after some time of you getting used to them you find them appealing?
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