¡Quiero amarte para siempre!

English Translation

I want to love you forever!

but why tho

I know! i thought ‘por’ meant duration. so it seem, I want to love you forever is about duration, and so it should be por, not para??

I quoted a longer answer to this question elsewhere and some considerable time ago. From what I remember, both por and para are acceptable in this example. The difference is: if you want to sound poetical you’d choose por; it has the sense of from the beginning of time to the end of time. However, in everyday speech, you’d use para.