What makes their bond so strong? Well, first and foremost, they've been teammates a long time. But loyalty goes a long way, and Crosby certainly remembers a time where Malkin displayed his loyalty to Sid.
A story came to light on Saturday from 2011 when Crosby was sidelined long-term due to a concussion. At the time, it was unclear as to whether or not Crosby would be able to resume his career and some players in the Pens' locker room were pushing for a leadership change. Malkin was having none of it, completely having Sid's back.
Knowing that Malkin had Crosby's back like that, it's no wonder why they're so close on and off the ice. That's the sort of friendship and loyalty you want from your teammates.