Forever is about murder

Seriously, what's the only thing that will last forever?
Death.

Let's take a look at the lyrics:

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If every word I said
could make you laugh
I'd talk forever

On the surface, this seems like a thing sweet thing to say. He wants to bring her laughter for the rest of their natural lives. Yes, how very nice. BUT YOU HAVE TO DIG DEEPER PEOPLE. The span of her natural life won't be much longer if she spends the entirety of it laughing. You see, her constant, continual laughter would deprive her of oxygen, her abdominal muscles would freeze, and she would perish. It's happened before, and he wants to make it happen again. There would be absolutely no signs of foul play, and Dennis would walk away a free man.

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I asked the sky
just what we had
It shone forever

Does this sound like a stable person to you? He was talking to the sun. On top of that, he presumably stood and stared at it for a period of time his mind registered as "forever," just to make sure it wouldn't stop shining.

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If the song I sing to you
could fill your heart with joy
I'd sing forever

I hope you're now starting to be able to pick up on the true, dark meaning behind these statements. This one should be clear enough. He doesn't want to make her joyful. Instead, he literally wants to fill her heart with joy. He's going to keep singing until her heart is entirely packed with joy, preventing it from beating and effectively shutting down the circulation of her blood. Once again, Dennis would go free, as he couldn't be blamed for her heart failure.

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I've been so happy loving you

Does anyone else notice that one, ominous word? "Been." He used to be happy with you, but it's over now. Now he is going to kill you.

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Let the love I have for you
Live in your heart
And beat forever

Maybe it's just me, but someone else's heart seems like a strange place to keep your things. I suppose once he removes all the joy from her heart cavities, there will be ample space. That word "live" is an interesting one, though. It's almost as if Dennis wants to reanimate her corpse. He's going to let his love live inside her heart, where it will begin to take over and resume its beating? For what nefarious reasons Dennis has to do this, I do not know.

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Baby just let me sing it my baby
I wanna be singin' my baby

The lyrics continue to mention singing--almost obsessively so. If we recall an earlier portion of the lyrics, singing is one of his plans to murder her. It seems he has chosen his method.

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So I'm goin' away
Mmm but not forever

It ends here--with a threat. It may seem as though he is gone, but he will be coming back. Is this a warning to the woman in the song? Perhaps.
Maybe, just maybe, it's a warning to the listener.