In Twilight why can't a mortal love a vampire?

In Twilight, a mortal cannot love a vampire because it is too dangerous. It is dangerous because the vampire may become too tempted to drink the human's blood. This is why Edward tried to resist being with Bella, even though he loved her more than anything else in his life. Edward thought that if he got near to Bella too much, he would kill her and turn her into a vampire forever- which wasn't easy. In Twilight, it was just too hard and too difficult for a vampire to love a mortal. Edward had to resist so much, even though he was trained not to drink human blood.