While the reason the Joker is the way he is was revealed after he received a superpowered upgrade courtesy of a being more chaotic than he could ever dream of being, and those true motives are pretty embarrassing, as they go against everything the Clown Prince of Crime claims to stand for.
In the crossover miniseries Joker/Mask by Henry Gilroy, Ronnie del Carmen, Howard Shum, and Ramon F. Bachs, the Joker gets his hands on the titular Mask (of Jim Carrey movie fame) and is granted all of the powers the artifact offers each wearer. The Mask grants anyone who wears it invincibility as well as the ability to manifest objects out of nowhere in a twisted cartoon-like fashion, all to fulfill their subconscious desires. Since the Joker is basically the poster child for chaotic excess, the Mask just enhances his already flamboyant persona while making him virtually unstoppable.
While the Joker doesn't change much after putting on the Mask in of personality, as opposed to previous wearers who were unrecognizable in their "Big Head" form, the Mask does uncover the Joker’s deepest desires and reveals the real reason he is a criminal mastermind. As Harley Quinn, Batman and others point out through the series, the Mask taps into the unconscious mind of anyone who puts it on and releases their pent-up desires. For the Joker, that pent-up desire is to run his own 24-hour broadcast filled with gags and criminality, constantly increasing the scope of his misdeeds to improve his ratings. This revelation proves that the Joker isn’t the Clown Prince of Crime because he just wants to create chaos for its own sake, but because he craves the world's attention and causing havoc is the best way to get it. Joker even holds off nuking Gotham city just so he can do it in primetime.
In this story, Joker isn't motivated by nihilism, a twisted love of Batman, or Grant Morrison's unexplored concept of his 'super-sanity.' Instead, he wants endless attention, employing unpredictable violence as a means to getting noticed. Of course, this doesn't stop him being deadly, especially since his already fractured mind prevents the usual process by which the Mask destroys its wearer. Joker is able to use the Mask's abilities to their fullest potential without any cost to himself, only removing it because Batman and Harley Quinn convince him that his audience is laughing at him, not with him.
Joker pretends to be driven by a dark philosophy in which nothing matters, explaining his crimes as an attempt to share this truth. However, Joker/Mask suggests that deep down, what the Joker really wants is to be noticed, and that his nihilism is a cover for a more personal desire he's far less willing to it.
