Why do people say, "I'll never take it off" then do it anyway?

People say those words figuratively, not literally. What the person’s expressing is, “I’ll always treasure this item,” or, “This gift is very dear to me.”

I’m with you, @Aster. I think the phrase is unfortunate, and I’d never say it. Gift-giving is a sensitive matter, especially when the donor and donee are very close. A person puts time, thought, effort, and love into buying something special, wanting it to be perfect. When the recipient opens the gift, the giver pays very close attention, in hopeful anticipation, and takes the reactions to heart. At a moment like that, excessive words such as, “I’ll never take it off!” are likely to be interpreted literally.

As with many words and phrases in the US, it has been gutted of true meaning, to just be that, mere words. I try to say what is truly meant, I myself would never say I would never take off, something someone gave me etc. because I know I actually will. I don’t say I will die for someone unless I truly in my heart would. People say they love each other all of the time but it is only conditional 90% of the time.

I think it is a figure of speech.
Sort of like when Samuel L. Jackson said, “If that motherf——er [Trump] becomes president, I’m moving my black ass to South Africa”.

If someone were really to go the rest of their lives without taking off an accessory like jewelry, it would become soiled from dirt, water, soap scum, etc. It’s not realistic to think someone would wear something for the next few decades.

To me, it’s a figure of speech. It’s not literal.