His feelings of lonliness and being out of place, not belonging, not being understood just caused him to retreat further back inside the walls he'd built to shield himself from the cruel world in which he'd been thrown. Jack was the only one in his life who treated him like a human being and with genuine kindness, understanding and loving him for who he is.
I am not sure if that is why Ennis was the lonesome cowboy that he was. I guess what you say contributed to his reticent and quiet life in the beginning, but in the rural west, people also respect personal choices, and would not presume to invade his privacy, his personal space, like LaShawna would surely do. Even Cassie's approaching him in the bar, would be considered very forward. Once Ennis adopted his lonesome cowboy persona after his couple years of high school, after the transmission on the truck broke, I think the "leaving him alone" attitude contributed to his shyness. In other words, I doubt that it was an active avoidance of the world, but rather a passive let the world be and let him be kind of non-communication. I don't think Ennis would be rude and refuse to talk to anyone. He was just the stoic cowboy.
What made Jack special to Ennis was his carefree attitude, and ability to draw him out, not invading his space, and willingness to listen, in addition to what you said, his respect for Ennis as equal and a human being, even though theoretically, being a little older and with a year of seniority, he could have bossed Ennis around, or dominated Ennis in conversation or whatever. Being treated respectfully and as an equal made Jack very attractive in the eyes of Ennis, to the effect of him devoting his life to Jack.
Unfortunately, like you said, the fear and restriction was too much and still deprived him of having the complete life he deserved, of being free and happy. He could only be free and happy in glimpses.
Yes, the homophobia instilled by his father at the age of nine, build the initial walls that isolated him from a happy life. And even with his lucky meeting with Jack, he was unable to overcome it.
And another thing I've wanted to add is that while it is debatable exactly at what age attractions/orientation become known, it is clear that it is not a concious choice the person makes.
As I said, there is insufficient information to make a determination, and any proposition is mere speculation. I have no trouble with other people making the choice to believe that Ennis was gay from birth. I personally choose to follow what Annie Proulx said, that they were two cowboys of indeterminate sexual orientation going up that mountain one summer, and came away in love with each other.
Without getting too off topic, since this discussion really belongs in the "Was Ennis Gay?" thread, and we are essentially re-stating what has already been gone over thoroughly there - if a person is not conscious of his sexual orientation, then is he gay or straight or bi? If a person can be attracted to men AND women, is he closeted gay, bi, or straight with a gay tendency? If a person is only unconsciously attracted to men, but does not do anything to pursue that attraction, and yet live happily with his woman, is he gay, bi, or straight?
These labels are fundamentally insufficient to describe the complexity of human sexuality and to force anyone to fit any one of these labels at a moment in time is to miss the bigger picture. Labels are made men, not men for labels.
To force a person to fit a particular label, in my opinion, is not only counter productive, it borders on prejudice.