PS When I first started posting on this forum (in the symbolism thread) I suggested that embodied in the story of Jack and Ennis are powerful archetypal images. Here is a summary of information I recently gleaned on the subject which may shed some light on why we all have been so powerfully affected in similar ways.
The philosopher Carl Jung believed that universal, mythic characters—archetypes—reside within the collective unconscious of people the world over. Archetypes represent fundamental human motifs of our experience as we evolved; consequentially, they evoke deep emotions.
Because an archetype speaks to the basic human experience, it communicates meaning and emotion in all forms of literature - from folklore to the classics. Since stories communicate truths about who we are (identity) and where we're going (destination), the most common archetypes are rendered through character and symbolism/situation.
One writer states that "A true encounter with an archetype is not only an experience of re-enchantment but also an experience which shakes us to our core".
Jung described the power of the archetype as "numinous" as it is experienced as having qualities of the divine.
IMO Brokeback Mountain tapped into the power of archetypal imagery (eg. Climbing the mountain; the union of lovers etc) which may explain the similar powerful responses we report experiencing.