I saw somewhere, can’t remember which site, that your planet controls your mood, like the moon controls the Earth’s tides. Or at least I think that’s what they meant.
Basically the Greeks (namely the astronomer Ptolemy), using astrological charts from the Babylonians, assigned each planet an archetypal character (or “energy”) represented by one of their gods, this archetypal energy is what influences the characteristic of your sun sign. There’s a much longer more informative version of this, look up the history of astrology if you’re curious it’s fascinating.