I think it is partly related to more permissive parenting styles that started in the 1960s, and there has been a snowball effect over the last few decades. Since the 70s, it has become fashionable to encourage kids to value themselves, but the part of the message that has faded is the part about also valuing and respecting others. Not to say that selfish behavior didn't exist previously, but since the 70s, it seems that school and popular media have specifically indoctrinated kids to be selfish. The intention is for kids to love themselves and think they are special, but the pendulum has swung too far, and we have a generation that to a disturbing degree lacks the capacity for critical self-evaluation.
Even though he was never big for frivolous on-field celebrations, I think Joe Namath's ostentatious persona with the Jets is what started us down this road. I think silly attention-seeking behavior was rare prior to Namath because guys were justifiably afraid of getting their ass kicked by other players, or even fans in the community. Namath was one of the first players to be so "important" and highly paid that he could afford to ignore the opinions of others. I think once one pro class does something, the next pro class is bolder about doing it too, or even going further "because I saw player X do it".
I think the conduct of players that kids see on TV is an even bigger influence. Players who are now 28 were old enough to know about "The Ickey Shuffle" in 1989, and before that there was the celebrated selfishness of Jim McMahon. Deion Sanders took it to a whole new level. A whole generation of players has grown up watching that conduct be not just tolerated, but rewarded. As players from the old guard have retired, any self-regulation that existed has faded even further. Soon the pro football ranks won't include anyone who is old enough to remember a time when thumping your chest and celebrating ordinary plays was not normal. A few jackasses paved the way, and now the kids who grew up watching them have turned it into the "new normal".