Is it not human nature to strive for more? For example, we all go to work each day, but dream about a better future for us, for our families. People work hard for promotions, go to college etc. It may not be utopia that we seek but we certainly hope for progression.
Applying the same theory to relationships, couples get married/become civil partners (in Scotland). People want more-more security, or, in Jack’s case, a commitment.
It was never mentioned in the film, but it appeared to me that during the twenty years previously, trips were never cancelled. The blow up occurred during their final meeting and neither mentioned the fact that trips were always being put off, simply that they weren’t long/often enough.
If that is the case, then Ennis gave Jack as much of a commitment that he could. On that basis, maybe Jack was thinking that love grows, children get older, society moves on and one day, one day….
Sandy
P.S. Wrangler, it did seem instantaneous and I thank you for explaining why! Sandy slaps forehead!