Pushing it where? It's more than a bromance and (for now) less than a romance. It's something BB hasn't seen before, in America at least. It's fascinating to a lot of people, especially since it's happening in real time on network TV every week.
Frankly I'd love to see it go even further, but I don't think Zach even knows what or who he is yet. He has claimed over and over again in interviews and in the DR that he doesn't really care about the other HGs, or people in general for that matter, and that he enjoys manipulating them. It sounds like he's so far in his own head when it comes to interpersonal relationships, he probably hasn't fully figured out yet what really turns him on.
He's said he really loves his family, especially his little brother, but Frankie is the only person in the house that he has stated in the DR that he actually cares about on a human level. For all we know, that could be a breakthrough for him. He's still a kid -- a precocious kid who a big mouth whose mom still takes care of him. It could be interesting to see who he turns into after this experience is over.