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I don't think we talk enough about what Mac falling for Johnny means in terms of his perception of love and his relationship to Dennis. I mean, we talk a lot about Dennis catfishing and e-dating him and the sexual component of it —which gives us plenty to unpack about Dennis' psyche, so it's understandable— but my favorite aspect to dissect is Mac's willingness to "fall in love" with a ghost.
In the episode, Dennis mentions there being texts between them so we know Mac wasn't just talking to a wall the whole time, but he's been stood up by Johnny so many times.. yet he keeps at it, keeps going on errands and to motels just in case Johnny decides to actually meet him this time. He's constantly waiting for someone who never shows up for him, and that's reflective of Mac's entire character. He keeps searching for love and validation from his mom, his dad, Dennis.. and when he doesn't get it, he just keeps on pushing, putting in more effort because maybe this time it will be enough. He holds so tightly onto his faith in god, a being he can't even see or hear, but that represents a hope for eternal love if you do everything right and conform nicely to its supposed expectations.
Mac isn't a stranger to loving distant beings, so of course he fell in love with Johnny. He's so desperate for someone to love him back but the only form of "love" he recognizes is a distant kind. That's why he can't give his date from the episode a chance, and why he doesn't want anything to do with uncle Donald. Easy and earnest love isn't something Mac knows. Hell, it isn't even worth it. Love, in his experience, requires work and sacrifice, otherwise what's the point?
This also plays a lot into his dynamic with Dennis, and why Mac will never let go of it. Sure, we saw him kind of trying to move on in 'The Gang Gets Romantic' by fabricating a love story for himself with Greg, and a more genuine attempt in 'Frank vs. Russia' by dating Johnny, but all roads eventually lead back to Dennis. A part of it can be attributed to Dennis not letting him move on —keeping Mac at arm's length while giving him just enough to keep him hooked— but that's not all. At this point I don't think there's anything Dennis could do or not do to put a definitive end to Mac's obsession with him, because he's exactly the type of person Mac craves in his life. As mentioned before, Dennis is someone that makes him earn his love and respect, and Mac can't find that in other potential romantic partners. He already tried looking for it online, where you can meet all kinds of people from all kinds of places, and who did he find? Dennis.
Obviously, the Johnny thing being orchestrated by Dennis means that Mac "found him" on purpose, but the point here is that Mac didn't know that. From his perspective, it was a fresh start with someone completely new, a guy that met his subconscious requirements. It just so happens that his requirements are Dennis. Because as mushy and gooey as it sounds, at the end of the day Mac is irreversably in love with him. He loves Dennis so intrinsically that he falls for him a second time, without even being aware of it. And I suspect he always will, no matter how many times he tries to move on.