it's a "trick" of the method of storytelling that was employed to suck us into the story and at a very deep level and to take on the emotions of the characters.
Chris makes a number of excellent points, especially the sentence above -- I think the power of BBM has much to do with effective storytelling. I also think there are a few other "tricks" in addition to the ones Chris mentioned.
One is that we're led to see Jack's and Ennnis' love as so great it's almost beyond the realm of ordinary human experience -- it's not a regular old pedestrian love like most people have, where you meet and date for a while and eventually maybe get more serious and blah blah blah. We are meant to see Ennis and Jack as almost superhuman when they're together -- soulmates, yin and yang, a force of nature, a pairing that belongs outside the confines of society and civilzation. Of course, there are practical reasons they exist outside society, mainly society's homophobia. But their association with mountains and water and sky and trees also serves to make their love seem more powerful and genuine and deep and pure.
In my view, another factor is we see how much each gives up to keep the love going -- in Ennis' case, it's being in love with Jack despite all his homophobia and shame and fear of being outed. For Jack, it's waiting for Ennis despite his disappointment and waning hope, missing Ennis so much he can hardly stand it, even after 20 years. They both struggle and sacrifice and suffer on behalf of their love (I know many would argue that Ennis doesn't sacrifice
enough, but actually for him it's a lot). The value and depth of their love is proved by what they are willing to give up for it.
Another storytelling trick is subtlety and restraint and ambiguity. Because the film doesn't always tell us what characters are thinking or why they behave the way they do, we're drawn into their heads, get more involved, empathize with them at a deeper level. Most movies go out of their way to show in no uncertain terms what their characters are feeling. Viewers have it easier, but they see the characters from a more objective distance. For example, take the scene where Jack doesn't turn to watch Ennis bathing. Most movies would show him quickly glancing over and trying not to be noticable about it. If BBM did that, it would tell us something -- but it would be something we already know at that point (he's attracted to Ennis), so we'd understand what he's thinking yet we'd stay detached. The fact that Jack
doesn't turn doesn't make us think he's not interested -- we already know he is. We have to make a greater effort to figure out why he does the opposite of what we'd expect, which makes us feel what he's feeling (temptation, self-restraint, frustration, bottled-up lust, whatever) at a deeper level. That same process happens again and again, every time a character acts ambiguously, which obviously is a lot, given that after all this time we're still discussing and arguing about it. What is Ennis thinking when he's eating pie in the bus station? Why didn't he respond to Jack's "miss you so much" comment? How does Jack feel about Randall's flirting? Was Jack really going to quit Ennis? The only way to answer these questions is by trying to understand the characters at a really deep level.
Compare some of these qualities to "Titanic" (that's the easy counter-example, being the other big popular recent romantic love story). Rose gives up something to go for Jack -- wealth and social status and her marriage -- but those sacrifices aren't as great as the ones made by Ennis' or BBM's Jack. We can see from the beginning she doesn't really value those things anyway. And Jack's only sacrifice to be with her is whatever risk he's taking of trouble from her husband.
Does Rose and Jack's love seem to exist outside normal human experience? Not really. It's outside
traditional social rules. And though they do seem well suited for each other, there's nothing really superhuman about them as a couple (of course, they're only together for a couple of days, compared to 20 years for Jack and Ennis).
Is their behavior ambiguous? Well, if Rose and Jack are feeling anything beyond what they're acting out onscreen, it sure went over my head. Their emotions seem pretty clear and obvious. We might like the characters fine, but we don't empathize with them as deeply as we do Jack and Ennis.
And it's sad in the end when Titanic Jack dies. But unlike Ennis, Rose has nothing to regret -- on the contrary, her life afterward is much better for having met Jack, so their story ends semi-happily.
To all that I would add that Heath and Jake do a
much better job of conveying passion and love than Leo and Kate do.
Of course, none of this explains why only WE appreciate these qualities about BBM when many others don't. That's still a mystery ...