Sire Bond

A Sire Bond is the link between the Sire and their Sired that exists within the vampire and hybrid species. The Sired complies with the direct and indirect orders of the Sire in order to show its loyalty.

Hybrids
Hybrids are loyal to their sire, and will seek their acceptance because their blood created them - this is called a "Sire Bond". Every hybrid is very loyal even when acting out of free will they can't stop doing what their sire says. Presumably, this also causes them to seek them out whenever their presence may be required.

Vampires
The link between two vampires is different than the link between two hybrids. Whereas the link between two hybrids is formed out of gratitude, the link between vampires seems to be formed out of love. The bond is at its strongest when the Sire is within the presence of the Sired.

When a human loves a vampire, and is turned by that vampire's blood, they may experience the sire bond, due to the emotions that they have felt being amplified by their transition, and do their best to please their sire. Hence, sire bonds between vampires normally aren't platonic, and usually common between those of opposing sexes. The sire bond does not influence feelings, only actions.

It may be possible to break the sire bond the same way hybrids do; hybrid sire bonds are based on gratitude and is broken when they have nothing to be grateful for when they turn over and over again. As long as hybrids are grateful to their master they will be sired; this could also mean as long as vampires are in love with their masters they will be sired. Vampire sire bonds are based on love and is possibly broken when they have nothing to love anymore, i.e. switching off their humanity. It is unknown whether or not the bond is maintained if the sired vampire "falls out of love" with their master.

Thus there are only two known ways to break a sire bond between vampires. The first is for the sire to tell the one bonded to them to forget about them and move on with their life. The other is for the sired vampire to turn off their humanity, which will break the sire bond. The sired vampire can do this themselves or be told by their sire to do it.

A third possible way was introduced in Season Four. It was alluded to, that if a vampire (be it the sire or the one sired) took the Cure, the sire bond between them would be broken (since presumably it would change the vampire into a human).