I always sorta thought to myself that Ennis and Alma met in High school or something...and because he was so shy, his brother and sister might have had a hand in encouraging him to settle down with such a 'nice girl'. Also, like David said, there is also a possibility that Alma pursued him, she clearly loved him from the start.
I have also never seen the writing on the door of the truck that Ennis gets out of in the beginning - nothing except Ennis exists in that scene, the way he hesitates abit, puts on his jacket and adjusts the collar, with a brown paper bag in his hand (holding all his worldly possessions probably)..*sigh*