capacitor?
Capacitor is a unilateral device. Those +/- signs are in electrolytic capacitors. This because electrolytic capacitors hold more charge. But how you make them? Most of them have one plate coated with Aluminium Oxide Al2O3 and also act as dielectric between plates.
Now if you have done some chemistry in high school in electrochemical reaction to happen here you would know this plate would become a positive sign of capacitor. Al2O3 most of time because looks like is the cheapest in market.
The reason is charge amount. The capacitor act as "unilateral" device in AC circuit but in DC exibiting (including those called pulsating DC) or pure DC circuits, watch how you connect the polarities.
In AC is not a problem.
It just chemistry forces us to make this stupid concession. In other hand we know the polarity electrolytic caps hold relatively large charge than any capacitor.
Most circuits today are AC, we love AC. Forget Edision with his DC. (kidding of course) In AC we are okay with any kind of capacitor.