i think KD is like depressed amongst other things. A lot of people don't really feel happiness or joy or excitement when depressed, especially if they're had it for a long time (having mental illnesses for long periods can actually change your brain apparently?). also, i feel like the only moments in his life where he was happy and having fun were the moments he got to spend with his grandma, because other than that he was probably busy studying/working his butt off.
It absolutely can change your brain. Mental illness are pretty often caused by imbalances of certain chemicals that your brain needs to function, so when that production gets thrown off for a long enough time some of the damage can be permanent. It's like letting an infected wound go septic; it can still likely be treated but the treatments will be extensive and exhausting and more often than not you will have left over permanent damage. So yeah if Dan has been depressed for a long time, on top of his substance abuse, he's got a long road ahead of him to get better.
It's a shame Dan isn't interested in anything. Jk was really into the game, and Dan, as usual, reacted like someone was trying to torture him...Maybe learning to swim will bring him joy and turn into a hobby. Also, have you seen Jk's blush? If Dan wears sexy, fashionable clothes, Jk will cum lol