"Tell her to live life without you and never think of you again - to stop caring about you.
And then leave her...." Witch in New Orleans
The sire bond is a sticky situation.
In one episode, we learned that sire bonds only form when the human was in love with the vampire before he/she turned (which, I'll admit, I'd already guessed). We also learned that there is a cure, but it comes at quite a cost.
The vampire has to tell his sired vampire to live without him/her....to stop thinking about the "master" vampire, and then the "master" has to leave.
All the while knowing that the sired vampire is truly in love with his/her master.
So I can stop with the awkward him/her stuff, I'm going to take it to a more basic level. Let's discuss the implications for Damon and Elena.
Damon already knew about sired vampires - we saw that way back when Tyler's bond to Klaus formed. He was aware of sire bonds because he'd sired another vampire. (What does that say about Damon that this "one in a million" occurrence had happened more than once?)
I'm fairly sure that Charlotte was a plot device - simply sired so that Stefan could witness (and to a lesser extent, Damon) the lengths of Charlotte's devotion. While both brothers were horrified that Charlotte had spent the last 40 years counting bricks, I'm pretty sure it meant different things to the two brothers.
Damon was hit by guilt, since he left after assuming the witch did what she said and had broken the sire bond. He didn't realize that Charlotte was waiting for him to return.
I'm pretty certain that Stefan saw a different picture. He focused on the extreme devotion Charlotte had for Damon. She was so intent on wanting to please Damon that she'd waited on the same street corner for 40 years. Stefan doesn't want to see Elena bound by that type of devotion - especially since (in my opinion), that's how he's rationalizing Elena's feelings for him changing to such a degree since she turned.
So, as the brothers leave New Orleans, Damon's task is clear. He has to free Elena. He has to tell her to stop caring about him - and leave. Stefan's convinced that Damon will do it because Damon can't be selfish with Elena.
But dealing with Elena's bond isn't as simple as Charlotte's.
Because he knows that Elena loved him while she was still human. She had to truly be in love with him before she turned for this to even become an issue. Becoming a vampire has given her the strength to look past all the criticism and condemnation from the rest of her "friends" and admit that she loves Damon. And now, to break that bond, he has to do the very thing he wants to avoid - he has to take away her choice to love him.
He promised never to leave her, but that's exactly what he has to do in order to break the bond.
So where does that leave him?
At some point, he's going to have to look into those deep brown eyes, the ones brimming with tears at the thought of losing him, and he's going to have to break the bond.
Because they can't stay in this honeymoon phase forever.
Damon and Elena can never be allowed to be this happy - and, honestly, have you ever seen either of them happier than at the opening of this episode (and I'm not just talking about the time in bed, I'm also addressing the "getting Elena ready for school" moments). Or at least they can't be this happy until the show's over or takes a substantially different direction.
Because Damon will never completely believe it's real for Elena.
We heard him admit it to her. He wants to know that everything she feels for him is real - otherwise, he would have compelled her a long time ago. Right now, he refused to break the sire bond because it's the right thing to do for her (to keep her from the confusion and grief evident on Charlotte's face), but at some point...that little hint of doubt that Elena doesn't really love him this much has to go away.
Because someone's probably going to use it against them.
I predict at some point, he's going to have to break the bond in order to save her. He loves her so much that he's going to have to let her go, no matter the amount of pain it will cause both of them.
The ultimate test will come when we watch to see if Elena will be able to overcome her "master's" orders to forget him and prove that her love for him is real.