in all honesty, it's usually because we're not on the same page, even if we were at the start (and sometimes, that may not even be the case). people change - it is what it is. sometimes goals change enough that all the caring in the world won't make up for it, and then it's time to move on. i wish more people understood this. i don't think it's *destined* to happen, but i think it happens more often than not. it's really a challenge (or maybe luck?) to find someone who's life goals change similarly enough over time to coincide with yours for years/decades. that, of course, doesn't mean you can't/shouldn't be able to share and cherish the time you have together, and part ways if/when the time comes, amicably at least, good friends (hopefully) at most.