Dan has a lot of similar characteristics to his dad, like being an alcoholic, being heavily reliant on people (jj or his grandma), being unable to pick himself back up.
Obviously Dan isn’t as bad as his dad but seeing someone living like his dad probably triggered jj to act more insensitively. Like he doesn’t want to get attached to another person that has the same self destructive habits as his dad.
Another thing, jjk probably saw a lot of his old “pitiful” self in Dan. Again triggering him. Jj is seeing his old self through Dan, but jj somehow feels superior because now he can look at it from a higher position.
It’s like when you see someone acting like how you used to before you matured and it icks you out, that’s basically what jj is seeing.
That does not even make sense, I thought JJ would be this reckless money spending guy because he already was born rich, and why he was so oblivious about why Dan was so money desperate in the beggining if he already lived that situation before???