Because they’re stupid as fuck, that’s my opinion.
They’re butthurt and feeling defensive (which means they’re feeling guilty) that they perceive his criticism to be reflective of them, since they know they pry too much into his life. And they criticize his personal life decisions, such as whom he dates, and it’s obvious Josh is upset by this—and instead of reflecting on why this might be, they lash out at him and act like he’s totally unreasonable for expecting some boundaries between him and his fans and that he’s a jerk for basically calling them (and the media) jerks. And these fans are extra salty about his love life because they want him to date Jen OR they have deluded themselves into thinking he could ever date them, or someone like them—i.e., a fan, because then that would give them hope for themselves. It happens all the time with fans and celeb crushes, and even if the fans never think they could actually date Josh, the crush still makes them possessive and jealous toward any other woman the guy dates.
Overall, these fans are just way too invested in his personal life and need to figure out how to take like two giant steps back and reevaluate their life decisions. Josh is the least problematic celebrity I’ve ever seen, honestly, and he certainly doesn’t deserve the level of vitriol and doubt that a lot of his fans are directing at him.