For me personally, I think its just the reminiscent feeling of BTS in their Youth, and how they represented such a time for younger people.
Also a lot of ARMYs will deny this and won't admit, but I personally feel there's a big guilt to those ARMY who joined late in BTS career, and came between 2018 to now.
Like they feel a little bad that they weren't their for BTS during their struggle days and before they had any fame and recognition.
So now every time they hear a song from BTS that's between 2016-2013, they just cry or feel this emotion thinking about BTS during that time. Plus the lyrics that was relatable and the MVs they made back then, that dealt with a bunch of realism.
WHAT'S YOUR OPINIONS AND THOUGHTS ON WHY THE HYYH ERA MAKES ARMY EMOTIONAL????