Narcissists MO - idealize, devalue, discard... It doesn't do any good to ask why, you will never really know why. Just know that they are disordered and that they will repeat this pattern with every person they enter into a relationship with, despite what they want you to believe. They will never change.
And your first love isn't always the one you're meant be spend your life with. You never truly get over this person because they are the one who taught you what it was to truly love someone. Although you may never fully get over it, you can move on. Sometimes the only way to get over someone to find someone who you care even more about. It may be hard to believe but when it happens, when they come along, everything seems to make a whole lot of sense.