Move Over, Carrie

Today's topic: At what point do you become responsible for another human being? 
This one's been percolating for a bit now. It may go around for a bit, but stick with me...we'll get there. You hear it a lot. "I told you not to expect anything." "It's your choice." "No one made that choice for you." "I don't owe you anything." 
I've heard it said from boys you have flirtatious flings with. They throw it out like a complete catch-all that will cover them should you try to push them for more than one night, surface-level conversation...once they say it--"I told you not to expect anything"--they act like they can get off Scott-free for the rest of your time together. 
But I've also heard it from boys who have made deep and grave promises to me. One boy, in particular. We had a ring, a date, a plan. And then he left me high and dry with no plans. Naturally what followed was a myriad of tearful conversations and screaming matches. And to this day, I will always remember the coldest smack in the face I ever received. After telling him how much he had hurt me and made me feel lost and alone with his action, he very quietly and simply said in the most even of carefree tones: "Why? Who am I?"
Who are you? You're the man I built my future plans with. You're my partner in this thing called love. You're my best friend. And even then, he wouldn't take responsibility for the fact that his actions hurt me. In his mind, the fact that I was unhappy and hurt was my choice and problem to deal with, not his. You made promises, I made choices. You broke those promises and now it's....my responsibility to pick up the pieces, while you have to deal with nothing?
Another example is from my mom and dad. Growing up, it was impossible not to notice that my father's mood always affected my mom's. How often do you remember making plans with one parent, only to have the other come home in a bad mood and have to cancel them? 
That's the type of responsibility I'm talking about. The ability to look someone in the eye and say: Who am I? I'm someone whose words, opinions and even moods influence the choices you  make. And I own up to that. You didn't make those choices alone. You were affected, by me. And that's a big deal. For both you and me. So I promise to be gentle and take care knowing that what I say carries extra weight. 
Which brings me to the question....at what point do you become responsible for another person and the way your actions affect them? If you move in with your boyfriend and it doesn't work out and you move out, does he in fact have some responsibility for the emotions that follow? Or, was it really "your choice" and therefor your responsibility to deal with the outcome, whatever it may be? 
If you make your choices based on input from other people,  what responsibility do they hold for the outcome of those choices?  In a world where no one takes responsibility,  is true love the best example of a self-sacrificing, pure miracle?  Move Over, Carrie is a whenever-I-feel-like-it series providing, hopefully, witty sociological commentary on modern relationship issues.