Sagittarius is the sign of freedom so basically it's very easy to break up with a Sagittarian which is to curb their freedom.
Being nagging, being needy, be someone who insists that they arrive on time, do what you want, can't go anywhere because you need them at home, it's very easy because Sagittarius needs to feel as if he or she not only is calling the shots, not in a controlling way but in a way of, "This is who I am, this is what I need, therefore I'm going out to get it".
If you stand in the way of that, then the Sagittarian quickly becomes annoyed. Also Sagittarius is very honest and again here, to be somewhat cagy or not entirely truthful about something or try to hide things from the Sagittarius is considered unforgivable.
Sagittarius also loves the great outdoors, loves animals. If you're mean to his or her dog or cat, if you don't want to go out and explore, if you don't want to go off swimming or go off dancing or go off rock climbing or whatever the Sagittarian likes to do, very quickly the Sagittarian will say, "I'm out of here".