I don't think you need an idea to write lyrics... you don't need a conscious reason...
Try this... If you have the chord progression, record it, give it plenty of bars so you have time to experiment.
Now play it back in headphones and record your voice, and rather than thinking what you want to write, just see how the music makes you feel and start singing STUFF... make sounds, nonsense words...go with the flow. It should come out as nonsense (avoid just jamming typical 'baby baby...love you... etc etc ... Go deep, just make sounds. Put the flow first.
Ok... now play it back, and write down what you think you might have been saying. e.g. if you sung "Wuderwul" maybe that's "Wonderful" or..maybe it's "What do I/we/you"... just write down what seems to suit best, don't overthink it.
Try to get just a few lines that way.... Read it back, and see what it might be about... again don't think too much, just get the general feel...and then start filling it out. i.e. you should be getting a theme about now, so you can fill in the gaps. Try to keep to the same syllables as the nonsense version. Keep building and refining... In the end you'll hopefully have something meaningful to you. Maybe it won't work for everyone...but it can work.
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Or... Try what Bowie learned from the beat poets... get several pages of words from books, newspapers, anywhere... and cut them up randomly (but in straight rectangles so you get 1 full word or maybe 2 or 3 words per cut. Vary it.) Actually a lot easier these days, just cut and paste on computer.
Put all your random words in a pile... and then begin putting them back together. Arrange them so that they tell part of a story or begin to say something meaningful to you...again fill in the gaps... think what the words are telling you, and correct them. You might not have many of the originals left when you're done, or you might have all of them. Also refine it to fit the phrasing/flow you need.
In the end you're still writing, still expressing yourself...it's coming from deeper within you and the restriction of the cuttings forces you to say things in a different way and talk about things you might not usually.
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So if you can't think what you want to say... try one of those methods. They do work. Plenty of very respected songwriters have used the cut-up method (Bowie, Cobain, Thom Yorke, to name a few...)
... Coming up with the music and melody is the harder part...
Dek