The following article contains spoilers from Fantastic Four #42, on sale now.
The friendship between Mister Fantastic and the Thing has endured through many rough patches. The most famous one of course is the Thing's anger about being permanently transformed following the fateful space flight that turned them into the Fantastic Four. The two have argued over it in the past, but recent years have shown that the Thing has moved past it. Reed is his best friend and nothing in the world would change that.
Fantastic Four #42 (by Dan Slott, Rachael Stott, Jesus Aburtov, Erick Arciniega, and VC's Joe Caramagna) however, showed that there was something Reed could do that might permanently end their friendship. To help Ben regain his fighting spirit, Reed lied to him about a failsafe killing Ben's wife and children. The resulting rage nearly pushed Ben to kill Reed for everything he's ever done to Ben, from turning him into the man he is today to costing him his future. It showed that Ben, despite statements to the contrary, still harbored those negative feelings towards Reed. Even after finding out that it was all a lie, Ben promised he would not forgive Reed for making him think he had lost his family, even if he was right.
This isn't even the first time in recent memory that Ben has aired his grievances against Reed. In Fantastic Four #30 (Dan Slott, Ze Carlos, Jesus Aburtov, and VC's Joe Caramagna) while briefly possessed by Knull, Ben attacked Reed, claiming that his so-called "best friend" does not care for them as much as he does for science. It's an argument that has been made about Reed before and one that only those closest to him could ever truly determine is true. Ben's argument was even after they were changed and thrown into so much danger, Reed never quit. He would continue to explore and put them in even more danger.
The two patched things up afterwards, but what had been said could not be unsaid. The thought would haunt both of them until it became a reality for Ben when Reed told him his wife and children were dead. At that moment, the rage was all that fueled Ben. He blamed Reed for his state of living, all the danger they've been put into, and how Reed had killed both their futures by dragging them off on adventures.
If it had all been true, Reed would be dead right now. Ben had managed to surprise him by exceeding his normal limitations when motivated by revenge for everything he had lost. Even after the ruse had been revealed, Ben still said he would never forgive Reed for that. He's not being unreasonable about this either. To prove a point and win a fight Reed put Ben through the worst kind of pain a husband and father could imagine, making him think that his family was gone forever. Ben had lost his past and future because of a mistake Reed made in the past. What made this time so different was that he had finally built a happy future for himself, only for Reed to intentionally destroy it this time.
The lie may have been necessary, Ben would need that unstoppable resolve to defeat the Reckoning, but it may have been the final nail in the coffin of their friendship. Ben has managed to forgive all of Reed's transgressions because they were largely byproducts of decisions they both made. Reed never forced Ben onto that ship or to come along on all his adventures, so any mistakes that were made could not fully be placed on his friend. This however was a choice Reed made to hurt him. He pretended that Ben had lost his future, so they could secure everyone else's. Even if this was the only way, it was still an unforgivable act.