You are such a good friend that if we were on a sinking ship together and there was only one life jacket. . . I'd miss you heaps and think of you often.

This is a darkly comic “backhanded compliment” that plays on the familiar setup of self-sacrifice in a crisis. The speaker begins with the language of deep friendship, then subverts expectations by implying they would keep the only life jacket for themself. The humor depends on abrupt reversal and exaggerated candor: affection is expressed not through altruism, but through the promise of fond remembrance after choosing self-preservation. As a joke, it can function as teasing intimacy—something said between close friends who understand the irony—while also satirizing sentimental declarations of loyalty by exposing the selfish impulse that can lurk beneath them.