Back then (2012), Jonghun even had many
outbursts - even on TV Shows like "Weekly Idol". Among Shawols he is known for this charactar trait.
He was very impulsive and gestured with his whole body - when he was happy or excited he jumped around, when he was surprised he had his mouth and eyes wide open, when he was angry he showed outbursts like the me tioned one. Back then, he seemed to be full of energy. On Shinee's One Fine Day (2012) he was laughing and hyped up the whole time and he was always the member who talked the most.
This changed abruptely. Jonghyun became calmer and more pensive, he didn't talk as much any more and started to share his emotions and thoughts on differnet (more serious and more phylosophical) topics. Even though the old side of him popped up from time to time, he never seemed to be the same as before.
Back in 2012 (or before?) the members stated that Jonghyun and Minho where the loudest backstage. In later interviews they told Jonghyun is much calmer backstage than he is on stage. He himself mentioned frequently (on Blue Night) how different the stage persona "Shinee's Jonghyun" is from the Jonghyun behind the scenes. And that he wanted to show his real self to the world.
Furthermore, there is an interview where the Shinee members said that they argued a lot and very loud in the past, because all of them where stubborn. However, later they learned to respect each other and even though they still discussed very loudly wouldn't feel offended. They said that outsiders might think that they where constantly arguing, even though they weren't.
Also, when Jonghyun made mistakes that caused controversions, he was always fast to react and wrote long and sincere excuses. He didn't even wait until SM give any statement, but posted them directly (like in the end of the year 2016).
I don't know what happened to him to change his behavior this much.
His dreams where constant over the years - he wanted to be a composer since debut days (and not a mere idol), showed self-produced songs on TV in 2012 and finally made a Solo Debut in 2015 after publishing many songs on his Blue Night Radio Show in 2014.
Something must have happened around 2013 for him to change that much.