Boros was an alien conqueror who toppled many worlds, unmatched in battle thanks to the raw power and strength he wielded.
Despite his great success though, Boros became very bored with his conquests, believing that without a worthy opponent there was no thrill. In truth, he only set his sights on Earth, because he believed there would be an opponent there that could finally give him a challenge.
Boros finally experience excitement as he got what he was looking for in an opponent, even though Saitama won that fight in the end. If Boros hadn’t met Saitama, he would’ve still been trapped in a never-ending cycle of easily conquering planets in search of the perfect challenge.