"I just can't do this" as an excuse to end a relationship.

It has come to light again recently that "I just can't do this." or "I can't do this to you." or "I just don't know." are now - a - days acceptable excuses for a rather serious relationship to end. How is it that you can go from talking about a future, living together, marriage, and children to one of these excuses so quickly.
News flash, everyone has issues at one point or another. The thing is being the adult and being able to discuss these things with your partner. When one side of the equation is left without understanding or resolution it is one of the hardest things to go through. Yes, you hit rough patches in your life, your mood and thinking change, but, isn't the point of being with someone the point that over time you're supposed to want to go through life with this person? I would say that yes, that is the point of dating someone and talking about a future. But what future is there really when you cannot talk about what bothers you with your partner.
Granted, it is not always the case that things work out 100%, but why not try? Instead of running away and thinking that your partner, girlfriend, boyfriend, etc -- doesn't understand, why not talk to h/er and figure things out. It makes no sense to 'fade out' (a term adapted from my own dying conversations through romance with people), or to just point blank run away from the person you're supposed to care about.
Since when has it become acceptable to run from your problems? I am quite sure that these excuses have not always existed in one form or another...but seriously?

What stops you from asking what is wrong when a conversation just seems wrong. One of the things which gets me, is that I am worried about starting something because of me trying to figure out what is going on. I know I don't always make it easy, but I know that trying is important. I, unfortunately, tend to pull away when I can feel a change in the person I'm seeing. This tends to be mainly when I'm talking to Joe. I get a little freaked about any move that I make because I never really know what makes us stop talking. Maybe someday we'll figure that out. How I would love to get it right someday.

*sigh* anyway. I feel bad for my little sister, who is currently in this predicament. Boys are weird.