Hi, ayashae, myprivatejack, thank you for the warm welcome back. It makes the feeling of home in this forum all the more real and poignant for me.

I agree with you both that we all do what Jack did when we are in love and try to win the love of the one we desire.
The difference I see between this honorable behavior and those who manipulate is that fine line when someone crosses over from good to evil. It is as Alexander Solzhenitsyn said in his Gulag Archipelagos that there is a fine line that divides every human heart, regardless of creed, politics, religion, sex, etc. We are all capable of good and evil. The difference is whether we act on the desire for selfish gains at the expense of other's suffering.
I think the difference between manipulation and winning the love of one's amore, is that we ultimately allow the person we love to choose, to decide, freely, once we have presented our case the best we could. We make the best impression, create the best atmosphere and mood, etc, but we leave the decision to the person. Evil manipulative people would go beyond that fine line, and use control and coercion to force whether physically or psychologically, even nowadays pharmaceutically, to submit to no choice at all. That, to me, is not love, but only selfish rape and abuse of body or soul.
I agree that Jack is not like that. I guess I am merely projecting what I have suffered myself onto what Ennis was going through after FNIT. His ride away from Jack, his encounter with the dead sheep as a parallel to his traumatic encounter with Earl, and finally, his redemption by hunting down the coyote. I think he felt unclean after the experience, and perhaps used, and that is why he refused to talk with Jack, until he performed a ritual of redemption. And then, after declaring his clear message on the mountain, he was ready for SNIT, but only hesitantly. It was only after he was in Jack's arms, feeling the comfort of his embrace, the sound of his soft whispers, and the gentleness of his caresses, that Ennis finally surrendered and trusted Jack, and gave in to his own feelings.
everybody, MPJ, Lance welcome back!
I've been reading the posts in this topic and it reminded me to one theme in the musical 'Evita'. I loved the soundtrack of the movie and used to listen to it pretty often those days. The chorus of that theme says:
"There is no one, no one at all
Never has been, and never will be a lover, male or female
Who hasn't an eye on, in fact they rely on
Tricks they can try on their partner
They're hoping their lover will help them or keep them
Support them, promote them
Don't blame them, you're the same" 
It doesnt mean that love is selfish or manipulative, because it's actually the opposite but, when we love someone (except the unconditional love from a mother, for example) we expect some feedback, and if we don't get it in some level, frustration can make us do crazy things, sometimes kinda 'selfish' (I don't speak about extreme behaviours which are always bad)
Take for example the scene of their last meeting, in the lake, when Ennis says 'i could kill you if I knew what you do in Mexico' 'it's because of you i'm like this' 'why don't you let me be' 'i can't stand this anymore' etc...Could we say Ennis tries to manipulate Jack? Maybe that would be a rational explanation of the situation, but we all know when it's about feelings, we humans are not 100% rational.
IMO both Jack and Ennis 'manipulate' each other, but it's human nature and out of love :\'( (but again I wanna emphasize it never justifies extreme behaviours)
I think that, ultimately, if we truly love someone, we put his happiness above our own, and accept the choice, however painful for us, if it meant less pain for them. It is what Jack did for Ennis, when Ennis said he could not take this any more, or in Sweet Home Alabama, when the bridegroom suffered public humiliation, yet, willingly let the woman he loved to return to the man she loved. True love means that the feedback from the other person will force us to accept the other person's feelings as our own, indeed, more than our own, and put their happiness before all else. That's what Ennis and Jack had. Their love is demonstrated in all the little acts of loving kindness throughout the movie, as others have already pointed out. (Thank you manhattangirl, flwrchild) The difference between selfish manipulation and selfless loving kindness is no where more clearly demonstrated than in their loyal relationship over twenty years.
So again, I apologize to all the Jack fans out there, and retract any implications that Jack was anything but honorable in his pursuit of Ennis' affection. His loyalty to Ennis for twenty years is proof enough of that.
Lance, welcome again¡.Manipulative or opportunistic...Aren´t we so-no matter how hard these words sound-when we´re in love? We take avantage of the opportunities we have for our benefit,it´s to say;to recover or get the one we love.Jack was a fighter in same way,he knew how to get what he wanted and had things very clear in this sense,and everything was leading to love,because above all he was a man in love.Nothing more than this. 
And yes,one of the best assets of the movie,maybe the best,is how it treats our feelings to get identified with the characters,no matter if we´re gay or straight,no matter if we´ve lived a similar experience or not...