Dick Grayson is the only former sidekick hero who has been trained by the Justice League, and his relationship with the JLA is different from his friends and siblings.
Green Arrow taught him how to use a bow, improving his accuracy long before Roy came along.
Oliver Queen spent time with him at galas Dick was forced to attend when Ollie was present. A few times he even took him with him to Queen Industries.
Aquaman taught him all styles of swimming, tricks for holding one's breath underwater, and Atlantean fighting moves for fighting in water.
Together with Mera, Arthur Curry took him to the best aquariums in the world, taught him the Atlantean language and told him some stories of Atlantis.
Martian Manhunter trained him to improve and strengthen his mental defenses.
J'onn J'onzz taught him the Martian language and told him some stories about Mars.
Black Canary helped him improve his hand-to-hand fighting style.
Dick Grayson and Dinah Lance enjoyed spending their time singing their hearts out.
Wonder Woman not only improved his hand-to-hand combat but was also the first person to teach him sword fighting, sword and shield, she taught him exclusively Amazonian fighting moves (and he is the only human who knows them) and later she introduced other weapons such as the lasso and others.
Diana Prince told him stories about Themyscira, taught him the Greek language (both ancient and modern) of the Amazons and her culture.
Green Lantern taught him all the tricks on how to perfectly (and even slightly reckless) fly a plane (Bruce never knew about their lessons).
Hal Jordan took him with him to work and with a big proud smile introduced him to his colleagues as his nephew.
Superman would take him with him and fly him around the world, stopping at every part of the planet Dick wanted to visit (including the Fortress of Solitude). Dick also learned how to hide and sneak up on meta-humans with super-hearing.
Clark Kent sometimes took him with him to the Daily Planet and even to lunches with the Kent family (much to Bruce's chagrin).
Hawkgirl improved his hand-to-hand combat style, taught him exclusively Thanagarian fighting moves (again, he is the only human who knows them), and introduced fighting with the spiked mace.
Shayera Hol taught him the Thanagarian language, told him stories about his home planet Thanagar and often flew him around with her.
Flash would put him on his shoulders and run around the world with him in a few minutes where yellow and red stripes would be seen followed by a mad giggle (all safely of course). Also, Flash helped Dick understand how to fight a speedster.
Although Bruce taught him the basics of chemistry, Barry Allen was his main science teacher and he taught Dick most of the science and chemistry topics he learned, even taking him with him at times to his job in the police crime lab proudly introducing him as his nephew. There is also no shortage of lunches with Barry and Iris West-Allen.
Dick Grayson was not only trained by the Justice League but also raised with them, and he knows that none of his friends or siblings have the same relationship with the JLA that he does.
And Dick may or may not be selfish, but he has always prided himself on the fact that he was the only one who regularly spent time with the JLA, training and growing up with them, and he wouldn't trade that for anything in the world.