There's really no way to explain Joker's motives or anything else. He is, as has been said, an Agent of Chaos. For various reasons and/or various ways he simply is what he is. Batman has made more than one attempt to try and get a sense of his thought-process, going so far as to lock himself in an isolation chamber and have a psychotic breakdown. Even THAT wasn't enough, to the point that Joker taunts him about it...
"You think it all breaks down into symbolism and structures and hints and clues? No Batman, that's just Wikipedia. You actually believed all it would take is a few chemicals, a couple of days of a drug-induced isolation and a cheap little nervous breakdown and you'd have ME all figured out? Like there was some rabbit-hole you could follow me down to understanding? HAH!"
The explanation about some men wanting to watch the world burn is probably the best concept for why Joker does what he does. Perhaps some kind of ultra-nihilism, but that's pure speculation. The bottom line is that he's an archetype, he's just enough character to be a character, but he's....well, to quote Spongebob of all people: "2% evil, 98% hot gas" in a way. He doesn't have a pinned-down personality or past and that's what makes him so versatile as a villainous construct. It's why he's lasted for so long in a semi-consistent way. Anyone can put words in his mouth because as long as it sounds rather mad and batman-obsessed, it never feels all that out of character. But such a lack of characterization also makes it difficult to see things like motives.
At one point I actually entertained the notion that the Joker was an Internet Troll before Internet Trolls existed. It makes sense in a way, but at the end of the day that's only because his character is so....loose...so to speak, that almost any motive or aspect of a personality can fit him.