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NRE is the term for new relationship energy, and that's the chemical state of mind that gives that 'puppy' state. It causes people to feel incredibly close, incredibly bonded, and people that haven't felt 'real' love before think that they are feeling love when they feel it, because it can be incredibly powerful.
But, the chemicals don't always release so strongly. And sometimes not at all. And even when they do release full blown, it is a temporary state. Sometimes it fades after a couple of months, sometimes a couple of years, and all stops in between. It's super rare for it to last more than two and half years, and 4 months may be a bit short, but 4 months isn't weird if the NRE didn't kick it very strongly.