It's a sad song, and maybe that's why Squalo likes it. On the cheap, scuffed mp3 player he bought from a pawnshop for less than 20 dollars, it's the only song. He plays it on a loop whenever he's traveling, having some free time, or just feeling melancholy. The end and the beginning connect seamlessly, playing infinitely until the swordsman's plane touches down, or Xanxus bursts in and tells him to get the fuck off his lazy ass and work, goddammit. He'll tuck it away into his pocket, or stuff it into a desk drawer to be more properly stowed at a later time, and forget about it.
Maybe because he lives a melancholy life, he's always reminded of that one song, that he returns to it again and again like an old sweater or pair of shoes that feels lived in, comfortable, and familiar. One anchor, one unchanging thing in his chaotic world, on an infinite loop.